This pan of fudgy marshmallow brownies is the bake I turn to when I want guaranteed smiles. I first made them for a neighborhood potluck after a string of rainy spring days, and the nostalgic combination of soft brownie, melty marshmallow, and quick-set chocolate icing had people hovering around the counter waiting for seconds. There is something irresistibly old-school about the way the marshmallows puff in the oven and then sink into the shiny icing. Every bite is a little stack of textures: chewy edges, dense chocolate middle, and a fluffy, sticky top. What makes these special is the balance of rich cocoa with chopped chocolate pieces that melt into pockets. The brownies start on the stovetop for an ultra-smooth base, and the topping comes together in minutes while the pan cools. I love that this dessert feels generous and festive without needing fancy equipment or decorating skills. You can cut small squares for a party tray, or larger pieces for a plate-and-fork treat after dinner. Either way, they never last long in my house.

Ingredients

• Unsalted butter: Forms the base of a glossy batter and helps deliver fudgy texture. Choose high-fat butter for best results and melt gently to avoid scorching.
• Granulated sugar: Dissolves into warm butter to create a shiny, cohesive batter that bakes up dense and moist rather than cakey.
• Canola oil: Adds tenderness and keeps the brownies soft for days; a neutral oil that won’t compete with the chocolate.
• Vanilla extract: Rounds out the chocolate flavor and enhances sweetness. Pure vanilla gives the warm aroma that pairs perfectly with marshmallow.
• Large eggs: Provide structure and richness. Add one at a time and beat well so the batter emulsifies and stays glossy.
• All-purpose flour: Just enough to set the brownies without making them cakey. Measure accurately and avoid overmixing.
• Dutch-process cocoa powder: Smooth, deep cocoa flavor with low acidity for that classic brownie color and taste.
• Salt: A small amount sharpens chocolate notes and balances the sweetness of the marshmallow and icing.
• Chocolate, chopped: Dark or milk chocolate cut into chunky pieces melts into pockets; thicker bars give satisfying bites.
• Mini marshmallows: Puff quickly in the oven and adhere to the brownie surface, creating a soft, gooey layer.
• Salted butter (for icing): Adds a balanced, slightly salty richness to the quick-set chocolate icing.
• Powdered sugar: Whisks into a silky icing that sets neatly for clean slices and a glossy finish.
• Unsweetened cocoa powder (for icing): Deepens the topping’s chocolate flavor without overwhelming sweetness.
• Whole milk: Loosens the icing to a pourable consistency that blankets the marshmallows.
• Vanilla extract (for icing): Adds warmth and bakery-style aroma to the final layer.

Instructions

Prep the pan and oven: Preheat oven to 350°F. Line a metal 9x13-inch pan with parchment, leaving overhang on two sides for easy lifting. Lightly coat the parchment and sides with nonstick spray. Arrange sheets so the shorter span sits under the full-length piece to prevent batter seepage. Emulsify butter and sugar: Melt 1 cup unsalted butter over low heat in a heavy saucepan. Stir in 2 1/2 cups granulated sugar until smooth and glossy, with no separated butter. Remove from heat and stir for 5 minutes to cool slightly and release steam, which protects the eggs from curdling. Add oil and vanilla: Stir in 2 tablespoons canola oil and 1 tablespoon vanilla until the surface looks uniform with no oily film. Proper emulsification creates a fudgy, cohesive crumb. Beat in the eggs: Add 5 large eggs one at a time, beating well after each addition. The batter should look thick and shiny. If it looks streaky, keep mixing until fully smooth. Fold in dry ingredients: Sift in 1 cup all-purpose flour, 3/4 cup plus 1 tablespoon Dutch-process cocoa, and 3/4 teaspoon salt. Fold gently just until the dry streaks disappear to avoid a tough, cakey texture. Add chocolate chunks: Fold in most of the 8 ounces chopped chocolate, reserving a small handful for the top. Thicker chunks melt into luscious pockets; avoid overmixing. Bake the brownies: Pour the batter into the pan, scatter remaining chocolate on top, and bake for 25 minutes. The center should look set at the edges with a slight wobble in the middle. Add marshmallows: Remove the pan and quickly blanket the surface with 6 cups mini marshmallows in an even layer. Return to the oven for 3 minutes to puff and adhere. Make the icing: In a saucepan over medium heat, melt 6 tablespoons salted butter. Whisk in 2 1/2 cups powdered sugar and 2 tablespoons unsweetened cocoa. Whisk in 1/4 cup whole milk and 2 teaspoons vanilla until smooth and pourable. Finish and set: Immediately pour the warm icing over the marshmallows, smoothing gently with a rubber spatula to cover. The icing will begin to set quickly. Cool completely before lifting out and slicing. Slice cleanly: Use a plastic knife for the cleanest cuts. For metal knives, lightly coat with nonstick spray and wipe between slices to prevent sticking.

Storage Tips

Cool completely, then store the brownies in a single layer or with parchment between layers in an airtight container at room temperature for 2 to 3 days. If your kitchen is warm, refrigerate for up to 5 days; bring to room temperature before serving for the best texture. For longer storage, freeze tightly wrapped slabs or individual squares for up to 3 months. Thaw overnight in the refrigerator or at room temperature for 1 to 2 hours. Avoid microwaving to reheat, as marshmallows can melt and get overly sticky; if you must, use very short bursts.

Ingredient Substitutions

No canola oil? Use melted neutral oil like vegetable or light olive oil in the same amount. If you only have natural cocoa, you can substitute cup-for-cup, but expect a slightly different flavor profile and a tad lighter color. Semi-sweet chocolate chips work in place of chopped bars; fold in 1 cup and reserve a few tablespoons for topping. If you’re low on powdered sugar, whisk 2 cups and add milk gradually until the icing is spreadable. For extra salt balance, use unsalted butter in the icing and add a pinch of salt to taste. Lactose-free milk works for the icing; plant milk can be used, but the set may be slightly softer.

Serving Suggestions

Cut small squares for party platters, or larger pieces with forks for a plated dessert. A scoop of vanilla ice cream or a dollop of lightly sweetened whipped cream plays beautifully with the warm chocolate notes. In summer, add sliced strawberries or raspberries on the side for a bright contrast. For birthdays, scatter a few sprinkles on the icing before it sets. At holiday gatherings, serve alongside coffee or hot cocoa so guests can enjoy the full marshmallow-chocolate experience.

Cultural Background

This style of brownie nods to the beloved Southern Mississippi Mud tradition, where marshmallows and warm chocolate icing form a soft, sticky “mud” over a rich base. Home bakers have passed versions of this dessert down for decades, often tweaking the topping or adding nuts. While every family has its spin, the constant is generosity: a big pan meant to share. The technique of adding a quick-set icing over warm marshmallows is a hallmark that transforms a simple brownie into a celebration treat with nostalgic charm.

Seasonal Adaptations

In fall, fold in 1/2 teaspoon cinnamon and top slices with toasted pecans for a cozy twist. Around winter holidays, add 1 teaspoon peppermint extract to the icing and sprinkle with crushed candy cane before it sets. In spring, swirl 1/4 cup raspberry jam over the marshmallow layer before pouring the icing. For summer cookouts, chill the sliced brownies and serve cold for neater bites in the heat; the icing stays snappy and refreshing.

Meal Prep Tips

Make the brownies a day ahead and keep covered at room temperature; the flavor deepens and the layers settle for cleaner cuts. For lunchbox portions, slice into small squares and wrap individually in parchment, then store in an airtight container. If freezing, place pieces on a sheet pan to firm up, then transfer to freezer bags to prevent sticking. Thaw wrapped pieces at room temperature to avoid condensation, which can make the icing tacky.

Whether you serve them at a potluck or just because it’s Tuesday, these fudgy marshmallow brownies bring the kind of joy that starts conversations and ends with happy, sticky fingers. Bake a pan, share generously, and make the moment a little sweeter.