Chocolate Mousse Brownies

Chocolate Mousse Brownies combine the dense chew of a classic fudgy brownie with a tall, cloud‑like layer of chocolate mousse, creating an impressive bakery‑style dessert bar that feels special enough for holidays but is simple enough for any weekend bake. The contrast between the dark, rich base and the light, creamy topping gives every bite layers of flavor and texture, especially when served chilled straight from the fridge.​

Ingredients

This recipe uses two main components: a deeply chocolatey brownie layer and a no‑fuss chocolate mousse made with melted chocolate and mini marshmallows. Quantities are designed for a 9×9‑inch square pan, which yields generous, bakery‑style squares.​

Brownie layer

• 8 oz semisweet chocolate baking bar, finely chopped
• 1/4 cup salted butter
• 1/3 cup vegetable oil
• 1 1/2 cups granulated sugar
• 2 large eggs, at room temperature
• 2 teaspoons vanilla extract
• 3/4 cup all‑purpose flour
• 1/2 cup unsweetened Dutch‑processed cocoa powder
• 1/2 teaspoon baking powder
• 1/2 teaspoon fine table salt​

Chocolate mousse topping

• 4 oz semisweet chocolate baking bar, finely chopped
• 1 cup mini marshmallows
• 1/4 cup plus 1 tablespoon whole milk
• 1 1/4 cups cold heavy cream
• Unsweetened cocoa powder or chocolate syrup, for garnish​

Room‑temperature eggs help the brownie batter emulsify smoothly, while high‑quality semisweet chocolate in both layers gives the bars a deep chocolate flavor that doesn’t rely solely on cocoa. Mini marshmallows in the mousse provide body and stability once chilled, so the topping slices cleanly and holds its shape without needing raw eggs.​

How to Make the Brownie Base

The brownie layer should be dense, moist, and intensely chocolatey so it can support the thick mousse on top. Using both melted chocolate and cocoa powder achieves that rich flavor and fudge‑like texture.​

1. Prepare the pan and oven

1. Heat the oven to 350°F (175°C).
2. Line a 9×9‑inch baking pan with parchment paper, letting the paper overhang two sides to form a sling. Lightly grease the parchment and any exposed pan edges.​

Lining the pan this way makes it easy to lift the entire slab of brownies out for clean slicing after chilling.​

2. Melt chocolate and fats

1. Place the 8 oz chopped semisweet chocolate and the 1/4 cup salted butter in a large, microwave‑safe bowl.
2. Heat in 20–30‑second bursts, stirring between each, until the chocolate is mostly melted.
3. When only small soft pieces remain, stop microwaving and stir until fully smooth.
4. Whisk in the 1/3 cup vegetable oil until the mixture is glossy.​

Melting the chocolate gently prevents scorching and keeps the mixture silky. The added oil contributes to that dense, moist chew associated with fudge brownies.​

3. Add sugar and eggs

1. Whisk the granulated sugar into the warm chocolate mixture until combined and slightly thickened.
2. Add the room‑temperature eggs one at a time, whisking thoroughly after each addition.
3. Stir in the vanilla extract.​

Incorporating the sugar into warm chocolate helps dissolve it partially, leading to a shinier crust and soft interior. Room‑temperature eggs blend more easily and help create a cohesive batter.​

4. Fold in dry ingredients

1. In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.
2. Sift or sprinkle the dry mixture over the wet batter.
3. Gently fold with a spatula just until no visible streaks of flour remain. Do not overmix, or the brownies can turn cakey.​

The combination of cocoa and melted chocolate intensifies flavor, while a small amount of baking powder gives just enough lift without sacrificing fudginess.​

5. Bake and cool

1. Pour the batter into the prepared pan and smooth the top.
2. Bake for 22–25 minutes, or until the edges are set and a toothpick inserted near the center comes out with moist crumbs but not wet batter.
3. Remove the pan from the oven and place it on a wire rack.
4. Cool the brownies completely in the pan; for the best mousse texture, they should be at room temperature or cooler before topping.​

Underbaking slightly keeps the brownies ultra‑moist, which is ideal since they will be chilled later; overbaked brownies can firm up too much in the refrigerator.​

How to Make the Chocolate Mousse

This mousse layer is light but stable, thanks to marshmallows and whipped cream. It sets up like a soft, sliceable cloud over the brownies, giving the bars their signature tall, velvety crown.​

1. Melt chocolate, marshmallows, and milk

1. In a medium saucepan over low heat, combine the 4 oz chopped semisweet chocolate, mini marshmallows, and milk.
2. Stir constantly as the marshmallows melt and the chocolate softens.
3. Continue stirring until the mixture becomes completely smooth and glossy, with no visible bits of marshmallow.
4. Remove the pan from the heat and transfer the mixture to a clean bowl.
5. Let it cool to room temperature, stirring occasionally; it should be thick but pourable, not hot, before mixing with whipped cream.​

Cooling is critical: if the chocolate‑marshmallow base is too warm when added to the cream, it can deflate the whipped texture or create streaks instead of a smooth mousse.​

2. Whip the cream

1. Place the heavy cream in a chilled mixing bowl.
2. Using a hand or stand mixer fitted with the whisk attachment, beat on medium‑high speed until stiff peaks form and the cream holds its shape when the whisk is lifted.
3. Take care not to overbeat; once the cream looks thick and slightly billowy, slow the mixer and watch closely so it doesn’t start turning grainy.​

Chilling the bowl and cream helps them whip faster and more stably, which is important for a mousse that needs to hold its shape on top of the brownies.​

3. Combine to make mousse

1. With the mixer on low speed, slowly drizzle the cooled chocolate‑marshmallow mixture into the whipped cream.
2. Once most of it is incorporated, stop the mixer and finish folding gently by hand with a spatula, scraping the sides and bottom of the bowl to ensure no streaks remain.
3. The mousse should be fluffy, smooth, and thick enough to mound slightly on a spoon.​

Folding gently preserves air bubbles in the cream, giving the mousse its airy texture, while the marshmallow base provides structure so it sets firm in the fridge.​

Assembling and Chilling the Brownies

Assembly is straightforward, but patience during chilling gives the best texture and cleanest slices.

1. Make sure the brownie layer is completely cool in the pan.
2. Spread the chocolate mousse evenly over the brownies, using an offset spatula to smooth the surface and reach all corners.
3. Cover the pan tightly with plastic wrap or foil.
4. Refrigerate for at least 2 hours, and preferably 4 hours or overnight, until the mousse is fully set and the bars are thoroughly chilled.​

Before serving, use the parchment sling to lift the chilled slab onto a cutting board. Dust the top lightly with unsweetened cocoa powder or drizzle thin lines of chocolate syrup for a decorative finish. Use a hot, dry knife (dip in warm water and wipe) to cut into neat squares, wiping the blade between cuts to preserve sharp layers.​

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