Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.

Melt the unsalted butter in a medium saucepan over low heat, stirring until completely melted. Remove from heat and stir in granulated sugar until glossy and smooth.

Let the butter-sugar mixture cool slightly, then whisk in the eggs and vanilla extract until smooth.

Sift together the flour, cocoa powder, salt, and baking powder in a separate bowl.

Gradually stir the dry mixture into the wet mixture, mixing gently until just combined.

Pour the brownie batter into the prepared baking pan, spreading evenly.

Bake for 30 to 35 minutes, until a toothpick inserted comes out with moist crumbs.

Allow the brownies to cool completely in the pan on a wire rack.

Melt the semi-sweet chocolate in a heatproof bowl over simmering water, then let it cool slightly.

Whip the heavy cream in a medium bowl until soft peaks form.

Beat the egg whites until foamy, then gradually add sugar and continue beating until stiff peaks form.

Fold the melted chocolate into the whipped cream, then incorporate the beaten egg whites.

Spread the mousse mixture over the cooled brownie base, smoothing into an even layer.

Chill in the refrigerator for 1-2 hours until set, then slice into squares and serve chilled.