Creating Chocolate Mousse Brownies is a straightforward process when you follow these steps:

For the Brownies:

1. Preheat Oven: Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking pan.
2. Melt Butter: In a saucepan, melt the unsalted butter over low heat. Remove from heat and stir in the granulated sugar.
3. Add Eggs and Vanilla: Add the eggs one at a time, stirring well after each addition. Then, mix in the vanilla extract.
4. Combine Dry Ingredients: In a separate bowl, sift together the flour, cocoa powder, salt, and baking powder.
5. Mix Together: Gradually add the dry ingredients to the wet mixture, stirring until just combined.
6. Bake: Pour the brownie batter into the prepared pan and spread evenly. Bake for 25-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs. Let cool completely.

For the Chocolate Mousse:

7. Melt Chocolate: In a heatproof bowl over a pot of simmering water, melt the chopped chocolate until smooth. Remove from heat and let cool slightly.
8. Whip Cream: In a mixing bowl, whip 1 cup of heavy cream until soft peaks form. Set aside.
9. Prepare Egg Yolks: In another bowl, whisk the egg yolks and granulated sugar until pale and thick. Stir in the melted chocolate and vanilla extract.
10. Fold in Cream: Gently fold the whipped cream into the chocolate mixture until fully combined.
11. Whip Egg Whites: In a clean bowl, whip the egg whites until stiff peaks form. Carefully fold the egg whites into the chocolate mousse mixture.
12. Spread Mousse: Once the brownies have cooled completely, spread the mousse over the brownie layer evenly.
13. Chill: Cover and refrigerate for at least 2-3 hours, or overnight for best results.