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

In a medium saucepan over low heat, melt the butter and chopped chocolate together, stirring until smooth. Remove from heat and let it cool slightly.

In a large mixing bowl, combine the granulated sugar, brown sugar, eggs, and 1 teaspoon of vanilla extract. Whisk until well blended.

Gradually stir in the melted chocolate mixture until combined.

Sift the flour and salt together, then fold it into the chocolate mixture until just combined. Do not overmix.

Pour the brownie batter into the prepared baking pan and spread it evenly.

Bake for 25–30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Allow the brownies to cool completely in the pan.

While the brownies are cooling, prepare the mousse. In a medium bowl, whip the heavy cream with the powdered sugar and 1 teaspoon of vanilla extract until soft peaks form.

Gently fold the whipped cream into the cooled chocolate mousse mixture until fully combined.

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

Refrigerate for at least 2 hours to set the mousse.

Cut into squares and serve chilled.