Preheat your oven to 350°F (175°C).

Melt the unsalted butter and chopped semi-sweet chocolate together over low heat in a medium saucepan. Stir continuously until smooth and well combined. Remove from heat and let cool slightly.

In a large mixing bowl, combine granulated sugar, brown sugar, eggs, and vanilla extract. Whisk until fully blended. Slowly pour in the melted chocolate mixture, stirring until smooth and creamy.

In another bowl, whisk together all-purpose flour, salt, and baking powder. Gradually add this dry mixture to the chocolate mixture, stirring gently until just combined.

Pour the brownie batter into a greased 9x13-inch baking pan and spread evenly. Bake for 25-30 minutes. Check for doneness with a toothpick.

While the brownies cool, melt the remaining chopped semi-sweet chocolate in a microwave-safe bowl in 30-second intervals, stirring until smooth. Allow to cool slightly.

In a large mixing bowl, whip heavy cream, powdered sugar, and vanilla extract until soft peaks form. Gently fold in the melted chocolate until fully combined.

Once the brownies have cooled completely, spread the chocolate mousse evenly over the top of the brownies.

Refrigerate the brownies for at least 120 minutes to allow the mousse to set. Cut into squares and serve cold.