Preheat your oven to 325°F (163°C) and grease a 9-inch round springform pan with cooking spray.

In a microwave-safe bowl, combine diced butter, 1 cup of granulated sugar, and cocoa powder. Microwave for 30 seconds, stir, then microwave for another 30 seconds.

Stir in salt and 1 teaspoon of vanilla extract. Allow the mixture to cool slightly before adding the eggs.

Beat in the eggs until well combined, then sprinkle the all-purpose flour on top and stir vigorously for about 50 strokes.

Pour the brownie batter into the prepared springform pan. Bake in the preheated oven for about 29 to 32 minutes until set yet slightly gooey in the center. Allow it to cool completely.

Beat 2 large eggs with the additional 1/4 cup sugar until pale, about 3-4 minutes.

Mix in half of the cold heavy whipping cream and transfer to a saucepan. Cook over medium heat while stirring until slightly thickened, about 3-4 minutes.

Remove from heat, add chopped chocolate, and whisk until smooth. Cool in the refrigerator for about an hour.

Beat remaining heavy whipping cream until soft peaks form, then gently fold in the cooled chocolate mixture.

Pour the chocolate mousse over the cooled brownie base in the springform pan and smooth the top.

Refrigerate for at least an hour to allow the mousse to set before serving.

Carefully remove the springform pan, transfer the mousse cake to a serving dish, and garnish with cocoa powder, chocolate shavings, or fresh berries.

Use a warm knife for clean slices and serve chilled, optionally with whipped cream or vanilla ice cream.