Preheat the Oven: Preheat the oven to 350°F (175°C). Grease and line an 8x8-inch baking pan with parchment paper.
Make the Brownie Layer: In a large bowl, combine melted butter, sugar, eggs, and vanilla extract. Stir in cocoa powder, flour, salt, and baking powder until fully combined. Pour the batter into the prepared baking pan and spread it evenly.
Bake the Brownies: Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. Let the brownies cool completely in the pan.
Make the Chocolate Mousse Layer: In a small saucepan, heat heavy cream over medium heat until it begins to simmer. Remove from heat and add chocolate chips. Stir until smooth and fully melted. Let it cool slightly.
Whip the Mousse: In a separate bowl, whip the cooled chocolate mixture with powdered sugar and vanilla extract until fluffy and mousse-like.
Assemble the Brownies: Once the brownies are completely cooled, spread the chocolate mousse layer evenly over the brownies.
Chill and Serve: Refrigerate the brownies for at least 2 hours or until the mousse is set. Slice and serve chilled.