Creating Chocolate Mousse Brownies is straightforward. Just follow these easy steps:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and line a 9×13 inch baking pan with parchment paper for easy removal.
2. Melt Butter and Sugars: In a medium saucepan over low heat, melt the unsalted butter. Add both granulated and brown sugars, stirring until smooth. Remove from heat.
3. Mix in Eggs: Allow the mixture to cool slightly. Whisk in the eggs, one at a time, followed by the vanilla extract until well combined.
4. Combine Dry Ingredients: In a separate bowl, whisk together the flour, cocoa powder, salt, and baking powder.
5. Combine Mixtures: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
6. Bake Brownies: Pour the brownie batter into the prepared baking pan and smooth it out evenly. Bake for 25-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
7. Cool Completely: Once baked, remove from oven and let the brownies cool in the pan on a wire rack.
8. Prepare Chocolate Mousse: In a heatproof bowl, combine the chopped chocolate and ½ cup of heavy cream. Melt in the microwave or over a double boiler, stirring until smooth. Let it cool slightly.
9. Whip Cream: In another bowl, whip the remaining heavy cream until soft peaks form. Gently fold whipped cream into the melted chocolate mixture until combined.
10. Whip Egg Whites: In another bowl, whip the egg whites until soft peaks form. Gradually add ¼ cup granulated sugar and continue to whip until glossy and stiff peaks form.
11. Fold Egg Whites: Carefully fold the whipped egg whites into the chocolate mousse mixture in three parts, being gentle to maintain the airy texture.
12. Assemble: Spread the chocolate mousse evenly over the cooled brownies.
13. Chill: Cover with plastic wrap and refrigerate for at least two hours, or until the mousse is set.
14. Serve: Once set, slice into squares and serve chilled.