Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.

In a medium saucepan, melt 1 cup of unsalted butter and 1.25 cups of semisweet chocolate chips over low heat.

In a large mixing bowl, whisk together 1 cup of granulated sugar, 4 large eggs, and 1 tablespoon of vanilla extract until light and frothy.

Slowly pour the melted chocolate and butter mixture into the egg and sugar blend, stirring constantly.

Gently fold in 1 cup of all-purpose flour, 0.5 teaspoon of baking powder, and 0.5 teaspoon of salt until just combined.

Spread the brownie batter into the prepared baking pan and bake for 25-30 minutes.

In a medium bowl, whip 3 cups of heavy cream, 0.25 cup of powdered sugar, and 1 teaspoon of vanilla extract until soft peaks form.

In a separate bowl, whip 2 large egg whites until stiff peaks form.

Fold the whipped cream into the stiff egg whites until well combined.

Spread the mousse evenly over the cooled brownies using a spatula.

Refrigerate the brownies for at least 2 hours to allow the mousse to set.

Once set, slice the brownies into squares and serve chilled.