Some desserts are made for quiet, cozy evenings, and these chocolate chunk brownies with a blanket of gooey marshmallows and a shiny chocolate frosting are exactly that kind of comfort. I stumbled on this combination after a summer barbecue when half a bag of mini marshmallows and a block of dark chocolate were staring back at me. The first batch disappeared before they even cooled, with everyone hovering in the kitchen waiting for the frosting to set. The contrast of fudgy brownie, stretchy marshmallow, and that glossy, soft-set icing has been a family favorite ever since.

What I love most is the texture symphony. The base is dense and fudgy with crackly edges, thanks to a warm sugar and butter mixture that dissolves just enough for a shiny top. Pockets of melty chocolate chunks bring extra richness in every bite. Then, a quick blast in the oven melts a layer of mini marshmallows into a soft, pillowy topping. The warm cocoa frosting cascades over the marshmallow, creating a satiny finish that gently firms as it cools. If you need a dessert that gets people leaning over the counter for seconds, this is the one I reach for every time.

Ingredients

• Unsalted butter: Richness and fudginess come from real butter. I like Kerrygold for its flavor. Cut into pieces so it melts evenly and emulsifies better with sugar.
• Granulated sugar: Domino sugar helps build that shiny, crackly top by partially dissolving in the warm butter. Don’t reduce too much or the texture changes.
• Dutch-process cocoa powder: Ghirardelli Dutch cocoa gives deep color and a smoother, less acidic chocolate flavor that complements the frosting.
• Large eggs: Vital Farms eggs add structure and chew. Add one at a time to keep the batter glossy and stable without overmixing.
• All-purpose flour: King Arthur AP flour offers dependable protein content for fudgy, not cakey, brownies. Measure accurately and avoid packing the cup.
• Chocolate chunks: About 8 ounces chopped; I like half a Trader Joe’s Pound Plus bar for varied chunk sizes and delightful melt pockets.
• Mini marshmallows: A generous layer melts into a gooey topping. Look for fresh, soft marshmallows so they melt evenly and stay pillowy.
• Powdered sugar: For the frosting’s silky body. Sifted to avoid lumps, it blends quickly with the cocoa and warm butter.
• Whole milk: Loosens the frosting to a pourable gloss. Room temperature milk prevents sudden seizing.
• Vanilla extract: A double dose in batter and frosting adds warmth and rounds out the chocolate notes beautifully.

Instructions

Prepare the pan and oven: Preheat the oven to 350°F. Line a metal 9x13-inch pan with parchment, leaving overhang for easy lifting. Lightly coat the parchment with cooking spray. Use two sheets if needed, with the shorter sheet underneath the long sheet for clean edges. Warm the butter and sugar: Melt 1 cup unsalted butter in a heavy saucepan over low heat. Stir in 2 1/2 cups granulated sugar until the mixture looks glossy and emulsified with no butter separating. Remove from heat and stir for 5 minutes to cool slightly; this helps dissolve sugar for a shiny top without scrambling the eggs. Add flavor and eggs: Stir in 2 tablespoons canola oil and 1 tablespoon vanilla until fully combined with no oily film. Beat in 5 large eggs one at a time, mixing just until incorporated. The batter should look thick, uniform, and shiny. Fold in dry ingredients: Sift 1 cup all-purpose flour, 3/4 cup plus 1 tablespoon Dutch-process cocoa, and 3/4 teaspoon salt over the pot. Gently fold until no dry streaks remain. Avoid overmixing to keep the texture fudgy and avoid tunneling. Stir in chocolate and bake: Fold in most of the 8 ounces chopped chocolate, reserving a handful for topping. Spread the batter evenly in the prepared pan and scatter remaining chunks on top. Bake for 25 minutes; edges will look set, and the center will be slightly soft. Add marshmallows: Pull the pan out and cover the surface evenly with 6 cups mini marshmallows. Return to the oven for 3 minutes, just until the marshmallows puff and soften but don’t brown. Transfer to a rack while you make the frosting. Make the frosting and finish: In a saucepan over medium heat, melt 6 tablespoons salted butter. Whisk in 2 1/2 cups sifted powdered sugar and 2 tablespoons unsweetened cocoa until smooth. Whisk in 1/4 cup whole milk and 2 teaspoons vanilla until glossy. Remove from heat and immediately pour over the marshmallows, spreading gently. Let set fully before lifting out and slicing.

Storage Tips

Cool the brownies completely before storing. Keep at room temperature in an airtight container for 2 to 3 days; parchment between layers prevents sticking. For longer storage, refrigerate up to 5 days, bringing slices to room temperature before serving so the frosting softens. To freeze, chill until the frosting is firm, then wrap individual squares tightly in plastic and place in a freezer bag for up to 3 months. Thaw overnight in the refrigerator or at room temperature for 1 to 2 hours. If the marshmallow layer becomes tacky, a brief chill helps reset the surface without drying the brownie.

Ingredient Substitutions

If you need a gelatin-free option, use vegan mini marshmallows; they melt a touch slower, so give them an extra minute in the oven. Swap canola oil with neutral avocado or vegetable oil in equal measure. For a different flavor profile, replace half the Dutch-process cocoa with natural cocoa for a brighter, slightly tangier chocolate note. Use semisweet or milk chocolate chunks, or a mix, keeping the total around 8 ounces. Gluten-free all-purpose flour blends can work 1:1; choose one with xanthan gum for structure and avoid overbaking. If you prefer a thinner frosting, add 1 to 2 teaspoons more milk until it pours in a steady ribbon.

Serving Suggestions

Serve these brownies slightly warm for maximum gooey appeal, or at room temperature for cleaner cuts and a soft-set frosting. A scoop of vanilla bean ice cream or a dollop of lightly sweetened whipped cream adds a cool counterpoint to the rich chocolate. For celebrations, drizzle warm caramel over the frosting or sprinkle with crushed toasted peanuts for a salty crunch. A dusting of cocoa or shaved chocolate adds a polished finish. Pair with strong coffee, cold milk, or a dark roast iced coffee to balance the sweetness.

Cultural Background

Brownies are a classic American bake, widely believed to have gained popularity in the early 20th century as a portable, bar-style dessert. The gooey marshmallow and frosting twist nods to Midwestern potluck traditions and church cookbook favorites, where simple ingredients meet crowd-pleasing presentation. The shiny, pour-over cocoa frosting recalls old-fashioned Texas sheet cake techniques, which rely on warm icing set over a warm base for a soft, nostalgic finish. It is a bake that bridges bake sale charm and holiday-table indulgence, familiar yet special.

Seasonal Adaptations

For summer cookouts, fold in chopped milk chocolate and top with crushed graham crackers for a s’mores vibe. In fall, add 1 teaspoon espresso powder to the batter to deepen the cocoa and sprinkle flaky salt on the frosting. Around the holidays, swap half the chocolate chunks for peppermint chips and finish with a sprinkle of finely crushed candy canes. In spring, swirl 2 tablespoons raspberry jam into the batter before baking; the fruit cuts the richness without overwhelming the chocolate.

Meal Prep Tips

Bake a day ahead for the cleanest slices; the frosting fully sets overnight. For grab-and-go portions, chill to firm, then cut into squares and wrap individually. Store in a lidded container to protect from fridge odors. Reheat a square in the microwave for 8 to 10 seconds to revive the marshmallow softness. If making for a party, bake in the morning, frost by midday, and slice right before serving for the neatest presentation and best texture.

Whether you are baking for a birthday, a potluck, or just a chocolate craving, these brownies deliver every time. Share the pan, savor the gooey middle square, and make them your signature sweet.