Preheat your oven to 325°F. Spray a 9-inch round springform pan with nonstick cooking spray.

In a microwave-safe bowl, combine the diced butter, 1 cup granulated sugar, and 3/4 cup cocoa powder. Microwave for 30 seconds, stir, then heat for another 30 seconds until the butter is melted and the mixture is smooth.

Stir in the salt and 1 teaspoon vanilla extract.

Add 2 eggs, mixing vigorously until the mixture is glossy and combined.

Sprinkle the flour over the top. Stir vigorously for at least 50 strokes.

Pour the mixture into the prepared springform pan, smoothing the top. Bake for 29–32 minutes until a toothpick inserted into the center comes out with moist crumbs. Let the brownie base cool completely in the pan.

Beat 2 eggs with 1/4 cup granulated sugar in a large bowl until pale and thick.

In a separate bowl, beat in half of the cold heavy cream (1 cup) until just combined with the egg mixture.

Pour this mixture into a saucepan and cook over medium heat while stirring constantly, until just thickened (3–4 minutes).

Remove from heat, add the chopped chocolate, cover, and let it sit for 1 minute. Whisk until smooth.

Chill the chocolate mixture in the refrigerator for about an hour until it’s cooled and slightly thickened.

In a clean bowl, beat the remaining 1 cup cold whipping cream until soft peaks form.

Break the chilled chocolate mixture into pieces and gradually mix it into the whipped cream. Add the final teaspoon of vanilla extract.

Spread the mousse evenly over the cooled brownie base in the springform pan. Refrigerate for at least an hour before serving.

Carefully remove the springform pan ring, slice, and serve your beautiful cake.