01 -
Preheat the oven to 350°F (175°C). Grease a 9×13 inch baking dish.
02 -
In a medium saucepan over medium heat, melt the butter. Stir in the sugar until thoroughly combined.
03 -
Remove from heat and allow to cool slightly. Whisk in the eggs and vanilla extract until fully blended.
04 -
Sift together all-purpose flour, cocoa powder, salt, and baking powder in a separate bowl.
05 -
Gradually fold the dry ingredients into the wet mixture, stirring just until combined; avoid overmixing.
06 -
Pour batter evenly into the prepared pan. Bake for 30-35 minutes or until a toothpick inserted near the center comes out with moist crumbs.
07 -
Remove from oven and cool completely in the pan before proceeding.
08 -
Using a double boiler or microwave, melt the semi-sweet chocolate until smooth. Allow to cool slightly.
09 -
Whip heavy cream in a large bowl until soft peaks form.
10 -
In a separate bowl, beat egg whites until foamy. Gradually add sugar and continue beating until stiff peaks form.
11 -
Gently fold melted chocolate into whipped cream until uniform. Then carefully fold in the egg whites, preserving airiness.
12 -
Spread the mousse evenly over cooled brownies. Cover and refrigerate for 1-2 hours to set before serving.