Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan or line it with parchment paper.

In a medium saucepan over low heat, melt the butter. Once melted, stir in granulated sugar and brown sugar until combined.

Allow it to cool slightly, then add the eggs and vanilla extract, mixing well.

In another bowl, whisk together flour, cocoa powder, and salt. Gradually add this to the butter mixture.

Pour the brownie batter into the prepared baking pan and spread evenly. Bake for 25-30 minutes or until a toothpick inserted comes out with a few moist crumbs.

In a small saucepan, heat the heavy cream over low heat until small bubbles form around the edges.

Remove from heat and add the chopped chocolate. Let it sit for a minute, then stir until smooth.

In a mixing bowl, beat the eggs and sugar until pale and fluffy. Gradually add the cooled chocolate mixture and vanilla extract.

Fold in any remaining whipped cream into the chocolate mixture gently until no streaks remain.

Once the brownies have cooled completely, pour the chocolate mousse over the brownie layer and smooth the top.

Chill in the refrigerator for at least 2 hours, or until the mousse is set.

Cut into squares and enjoy your delicious chocolate mousse brownies!