Preheat the Oven: Preheat your oven to 350°F (175°C) and grease a square baking dish or line it with parchment paper for easy removal.
Make the Brownie Batter: In a medium saucepan, melt the butter over low heat. Once melted, remove from heat and stir in the sugar, eggs, and vanilla until well combined.
Add Dry Ingredients: Next, mix in the cocoa powder, flour, salt, and baking powder until just combined (don't overmix!).
Bake the Brownies: Pour the batter into your prepared baking dish and spread it evenly. Bake for about 20-25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Cool the Brownies: Allow the brownies to cool completely in the baking dish before adding the mousse layer.
Prepare the Mousse: In a mixing bowl, whip the heavy cream, powdered sugar, and vanilla until soft peaks form. Carefully fold in the melted chocolate until fully incorporated.
Layer the Mousse: Spread the chocolate mousse evenly over the cooled brownies using a spatula. Chill in the refrigerator for at least 2 hours to set the mousse.
Serve and Enjoy: Once the mousse has set, slice your brownies and serve. Enjoy!