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

In a saucepan, melt the butter over medium heat.

Remove from heat and stir in both sugars until well combined.

Add the eggs one at a time, mixing well after each addition.

Stir in the vanilla extract.

In a separate bowl, whisk together the flour, cocoa powder, and salt.

Gradually stir the dry ingredients into the wet mixture until combined.

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

Let cool completely.

In a heatproof bowl, combine the heavy cream and chopped chocolate. Microwave in short intervals, stirring until smooth.

Allow to cool slightly before spreading over the cooled brownies.

Chill the brownies until the mousse is set, then cut into squares and serve.