Creating these Chocolate Mousse Brownies is incredibly rewarding. Follow these steps for a successful outcome:

Step 1: Preheat the Oven

Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking pan with butter or line it with parchment paper for easy removal.

Step 2: Prepare the Brownie Batter

1. In a medium saucepan, melt the butter over low heat.
2. Once melted, remove from heat and stir in the granulated sugar until well combined.
3. Add the eggs, one at a time, mixing well after each addition.
4. Stir in the vanilla extract.
5. Sift together the flour, cocoa powder, and salt in a separate bowl.
6. Gradually incorporate the dry ingredients into the wet mixture, stirring until just combined.

Step 3: Bake the Brownies

Pour the brownie batter into the prepared baking pan, smoothing the top with a spatula. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Allow baking for the brownies to cool completely in the pan.

Step 4: Make the Chocolate Mousse

1. Melt the chopped semi-sweet chocolate in a heatproof bowl set over simmering water, stirring constantly until smooth and combined. Remove from heat and let it cool slightly.
2. In a separate bowl, whip the heavy cream until soft peaks form. Set aside.
3. In another bowl, whisk the egg whites until foamy; gradually add the sugar while continuing to whisk until stiff peaks form.
4. Fold the melted chocolate into the whipped cream gently until no streaks remain.
5. Finally, fold in the whipped egg whites, being careful not to deflate the mixture.

Step 5: Assemble the Brownies

1. Once the brownies have cooled completely, add a layer of chocolate mousse on top of the brownies, spreading it evenly.
2. Cover and refrigerate for at least 2 hours to let the mousse set properly.

Step 6: Serve and Enjoy

Once chilled and set, you can cut the brownies into squares. They should hold their shape nicely.