Chocolate Mousse Brownies

Preheat your oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper, leaving an overhang on the sides. In a heatproof bowl set over a saucepan of gently simmering water (double boiler method), melt the unsalted butter and chopped semi-sweet chocolate together, stirring occasionally until smooth and glossy. Remove from heat and let cool slightly.

Add granulated sugar and brown sugar to the melted chocolate mixture. Whisk until thoroughly combined. Add eggs, one at a time, whisking well after each addition. Stir in vanilla extract. Sift in all-purpose flour, unsweetened cocoa powder, and salt. Use a spatula to gently fold the dry ingredients into the wet mixture until just combined—do not overmix.

Pour the prepared brownie batter into the lined 8-inch square baking pan. Use a spatula to spread the batter evenly to the edges and smooth the top, ensuring the layer is uniform for even baking.

Place the pan in the preheated oven and bake for 30–35 minutes, or until a toothpick inserted into the centre comes out with a few moist crumbs (not wet batter). Remove the pan from the oven and set it on a wire rack. Let the brownies cool completely in the pan—this step is essential for the mousse layer to sit correctly.

Place the chopped semi-sweet chocolate in a heatproof bowl. In a small saucepan, heat half of the heavy whipping cream (120 ml) over medium heat until it just begins to simmer (do not boil). Pour the hot cream over the chocolate and let it sit for 2 minutes to soften. Then, gently whisk until the chocolate is completely melted and the mixture is smooth. Allow this chocolate ganache to cool slightly to room temperature.

After the hot cream has sat on the chopped chocolate for 2 minutes, use a whisk to gently stir the mixture until the chocolate is completely melted and the ganache is smooth and shiny. Set the bowl aside and let the ganache cool to room temperature before continuing.

Pour the remaining heavy whipping cream (120 ml) into a chilled mixing bowl. Using a hand mixer or stand mixer fitted with the whisk attachment, beat the cream on medium-high speed until it starts to thicken. Add granulated sugar and vanilla extract, then continue whipping until medium-stiff peaks form—be careful not to overwhip. The cream should be fluffy but hold its shape.

Once the chocolate ganache has cooled to room temperature, add the egg yolk and whisk until fully incorporated. Gently fold about one-third of the whipped cream into the ganache to lighten it. Then, carefully fold in the remaining whipped cream in two additions, using a spatula and making broad, gentle strokes. Continue folding until the mousse is smooth and thoroughly combined, being careful not to deflate the whipped cream.

Once the brownie layer has completely cooled in the pan, gently spread the chocolate mousse evenly over the top using a spatula. Smooth the surface so the mousse layer is level and reaches all corners. Cover the pan tightly with plastic wrap and refrigerate for at least 2 hours, or until the mousse is set and firm to the touch.

Once the mousse is fully set, remove the pan from the refrigerator. Carefully lift the brownies out of the pan using the parchment handles and transfer them to a cutting board. Peel away the parchment and use a sharp knife to slice into squares, cleaning the knife between cuts for neat edges. Garnish each brownie with shaved chocolate, a dusting of cocoa powder, or fresh berries as desired. Serve chilled for best texture.