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

Melt ½ cup of unsalted butter in a saucepan and stir in 1 cup of granulated sugar until combined.

Remove from heat and whisk in 2 large eggs and 1 teaspoon of vanilla extract until smooth.

In a separate bowl, combine 1 cup of all-purpose flour, ⅓ cup of cocoa powder, ½ teaspoon of salt, and ½ teaspoon of baking powder; sift together.

Gradually fold the dry ingredients into the wet ingredients until just combined.

Transfer the batter into the greased baking pan evenly.

Bake for 30-35 minutes until a toothpick comes out with a few moist crumbs.

Allow the brownies to cool completely in the pan for about 30-45 minutes.

For the mousse, melt 8 ounces of semi-sweet chocolate and let it cool slightly.

Whip 1 cup of heavy cream until soft peaks form.

Beat 2 egg whites until foamy, then gradually add 2 tablespoons of granulated sugar until stiff peaks form.

Gently fold melted chocolate into whipped cream, then fold in the egg whites until combined.

Spread the mousse evenly on the cooled brownies.

Chill the brownies in the refrigerator for 1-2 hours before serving.