Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper for easy removal. In a large mixing bowl, whisk together melted unsalted butter, granulated sugar, large eggs, and vanilla extract until smooth and glossy.

Gradually sift in unsweetened cocoa powder, all-purpose flour, salt, and baking powder, folding gently. Pour the dense brownie batter into the prepared pan and bake for 20–25 minutes, or until a toothpick inserted in the center comes out with moist crumbs. Let cool completely.

In a small saucepan, heat milk over medium heat until it just starts to steam, then remove and pour over finely chopped semisweet chocolate. Let sit for 2 minutes, then stir until smooth. In another bowl, whip chilled heavy cream with powdered sugar until soft peaks form. Mix a spoonful of whipped cream into the chocolate, then fold in the remaining whipped cream. Spread over cooled brownies.

Heat additional heavy whipping cream in a saucepan until simmering, then pour it over semisweet chocolate in a bowl. Let it sit for 2-3 minutes, then stir until glossy and smooth. Pour the ganache over the mousse layer, smoothing with an offset spatula. Chill in the refrigerator for at least 1 hour until the ganache is set.

Once chilled and set, use a warm knife to cut into squares. Serve chilled, garnished with whipped cream and fresh raspberries if desired.