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

In a large bowl, combine melted butter, sugar, cocoa powder, and salt. Mix well.

Stir in eggs one at a time, then add vanilla extract. Gradually add flour and mix until just combined.

Pour batter into the prepared pan and bake for 20-25 minutes, or until a toothpick inserted into the center comes out with moist crumbs.

Let cool completely before adding the mousse layer.

Melt chocolate in a heatproof bowl set over a saucepan of simmering water (or in the microwave in 30-second intervals, stirring in between).

Remove from heat and stir in 1/2 cup of heavy cream until smooth.

In a separate bowl, whisk egg yolks with sugar until pale and thick. Gradually whisk in the melted chocolate mixture.

In another bowl, beat egg whites until stiff peaks form. Gently fold into the chocolate mixture.

In a separate bowl, whip the remaining 1/2 cup of heavy cream until stiff peaks form. Gently fold into the chocolate mixture.

Pour the mousse over the cooled brownie layer and refrigerate for at least 2 hours before serving.