Creating Rich & Fudgy Mousse-Topped Brownies is straightforward if you follow these steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). 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 and stir in granulated sugar, eggs, and vanilla until smooth.
3. Mix Dry Ingredients: In a separate bowl, whisk together flour, cocoa powder, salt, and baking powder.
4. Combine Mixtures: Gradually add the dry ingredients to the wet mixture. Stir until just combined; do not overmix.
5. Bake the Brownies: Pour the brownie batter into the prepared pan. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
6. Cool the Brownies: Remove from the oven and allow the brownies to cool in the pan for about 15-20 minutes. Then, transfer to a wire rack to cool completely.
7. Prepare the Mousse: In a small saucepan, heat the heavy cream over low heat until it begins to simmer. Remove from heat and add the chopped chocolate. Stir until smooth and let cool slightly.
8. Whip Egg Whites: In a clean bowl, beat the egg whites until soft peaks form. Gradually add sugar and continue to beat until stiff peaks form.
9. Combine Mousse Ingredients: Gently fold the cooled chocolate mixture into the whipped egg whites. Be careful not to deflate the mixture.
10. Top the Brownies: Once the brownies are completely cool, spread the mousse evenly over the top.
11. Chill: Refrigerate the brownies for at least 2 hours before serving to allow the mousse to set.