Preheat your oven to 325°F (160°C) and lightly grease a 9-inch round springform pan with cooking spray.

In a microwave-safe bowl, combine the diced butter, sugar, and cocoa powder. Microwave on HIGH for 30 seconds, then stir and microwave again for another 30 seconds.

Once melted, stir in the salt and vanilla extract. Add the two large eggs, mixing until smooth. Then, sprinkle the flour on top and stir vigorously for about 50 strokes until fully combined.

Pour the brownie batter into the prepared springform pan and bake for 29-32 minutes. Let it cool completely before proceeding to the mousse layer.

To prepare the mousse, beat the two large eggs with the 1/4 cup of sugar in a bowl for 2-3 minutes, or until the mixture is pale yellow.

Gradually pour half of the cold heavy cream into the egg mixture. Cook on medium heat while stirring until it starts to thicken (do not allow it to boil).

Remove from heat, add the chopped chocolate, and cover the bowl. Let it sit for 1 minute, then whisk until smooth. Chill this mixture in the refrigerator for about an hour.

In another bowl, beat the remaining heavy cream until soft peaks form. Gently fold in the chilled chocolate mixture until just combined.

Pour the mousse over the cooled brownie in the springform pan, spreading it evenly. Refrigerate for at least one hour before serving.

When ready, release the springform pan and slice the cake to reveal the beautiful layers.