Creating Rich & Fudgy Mousse-Topped Brownies is a delightful process. Follow these steps to achieve chocolate perfection:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and line a 9×13-inch baking pan with parchment paper.
2. Melt the Butter: In a medium saucepan, melt the unsalted butter over low heat. Remove from heat and stir in the granulated sugar until combined.
3. Add Eggs and Vanilla: Allow the mixture to cool slightly, then whisk in the eggs, one at a time, followed by 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 fold the dry ingredients into the wet mixture until just combined. Be careful not to overmix.
6. Bake the Brownies: Pour the brownie batter into the prepared baking pan. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
7. Cool the Brownies: Once baked, remove the brownies from the oven and let them cool in the pan on a wire rack.
8. Prepare the Chocolate Mousse: While the brownies are cooling, melt the chopped chocolate in a heatproof bowl set over a pot of simmering water. Stir until smooth and let it cool slightly.
9. Whip the Cream: In a large bowl, whip the heavy cream until stiff peaks form. Set aside.
10. Combine Egg Yolks and Sugar: In another bowl, whisk together the egg yolks and granulated sugar until pale and thick.
11. Combine Chocolate and Yolks: Stir the melted chocolate into the egg yolk mixture. Gently fold in the whipped cream until fully incorporated.
12. Beat Egg Whites: In a clean bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar and beat until glossy.
13. Fold in Egg Whites: Gently fold the beaten egg whites into the chocolate mixture until no white streaks remain.
14. Top the Brownies: Once the brownies are completely cool, spread the chocolate mousse evenly on top.
15. Chill: Cover and refrigerate for at least 2 hours or until the mousse is set.
16. Slice and Serve: Once chilled and set, slice the brownies into squares and serve.