Making Chocolate Mousse Brownies is a straightforward process if you follow these well-organized steps:

For the Brownies

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking pan.

2. Melt Butter and Sugar: In a medium saucepan, melt the butter over medium heat. Once melted, stir in the sugar until well combined.

3. Add Eggs and Vanilla: Remove from heat and let it cool slightly. Whisk in the eggs and vanilla extract until smooth.

4. Mix Dry Ingredients: In a separate bowl, sift together the flour, cocoa powder, salt, and baking powder.

5. Combine Mixtures: Gradually add the dry mixture to the wet mixture, stirring just until combined. Do not overmix.

6. Pour Batter: Spread the brownie batter evenly into the prepared baking pan.

7. Bake: Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

8. Cool: Once baked, remove from the oven and allow to cool completely in the pan before adding the mousse layer.

For the Chocolate Mousse

9. Melt Chocolate: In a heatproof bowl, melt the chopped chocolate over a double boiler or in the microwave. Stir until smooth, then let it cool slightly.

10. Whip Cream: In a large mixing bowl, whip the heavy cream until it forms soft peaks.

11. Beat Egg Whites: In another bowl, beat the egg whites until foamy. Gradually add the sugar, continuing to beat until stiff peaks form.

12. Combine Mixtures: Fold the melted chocolate into the whipped cream until well combined. Then gently fold in the beaten egg whites, being careful not to deflate them.

13. Spread Mousse: Once the brownies are completely cooled, spread the chocolate mousse evenly over the top.

14. Chill: Cover the brownies and refrigerate for at least 1-2 hours, allowing the mousse to set.