Creating these incredible Chocolate Mousse Brownies is easier than you might think! Follow these steps carefully:

For the Brownies:

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking pan.
2. Melt the Butter: In a medium saucepan, melt the butter over low heat. Once melted, remove from heat.
3. Add Sugar and Eggs: Stir in the sugar until well combined. Add the eggs, one at a time, beating well after each addition, then mix in the vanilla extract.
4. Combine Dry Ingredients: In a separate bowl, whisk together the flour, cocoa powder, salt, and baking powder.
5. Mix the Batter: Gradually add the dry ingredients into the wet mixture, stirring until just combined. Do not overmix.
6. Pour into Pan: Pour the brownie batter into the prepared baking pan, spreading it evenly.
7. Bake: Bake for 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Allow to cool in the pan.

For the Mousse Topping:

8. Melt Chocolate: Place the chopped chocolate in a heatproof bowl. In a small saucepan, heat ½ cup of the heavy cream until it just simmers. Pour it over the chocolate and let it sit for 2-3 minutes. Stir until smooth and let it cool slightly.
9. Whip Cream: In a mixing bowl, whip the remaining ½ cup of heavy cream until soft peaks form. Set aside.
10. Beat Egg Whites: In another clean bowl, whip the egg whites until soft peaks form. Gradually add the sugar and continue beating until stiff peaks form.
11. Combine Mixtures: Gently fold the whipped cream into the cooled chocolate mixture, then fold in the egg whites until no white streaks remain.
12. Spread Mousse: Once the brownies have cooled, spread the mousse evenly over the top of the brownies.
13. Chill: Refrigerate for at least 1 hour, or until the mousse is set.