Preheat your oven to 350°F (175°C). Grease an 8x8-inch baking pan or line it with parchment paper for easy removal.
In a medium saucepan over low heat, melt the butter and dark chocolate chips together, stirring until smooth. Remove from heat and let it cool slightly.
In a large mixing bowl, whisk together the granulated sugar and eggs until combined. Add the melted chocolate mixture and vanilla extract, mixing until well combined.
Sift the flour and salt into the chocolate mixture, folding gently until just combined. Be careful not to overmix.
Pour the brownie batter into the prepared pan, spreading it evenly. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Let cool completely in the pan.
In a separate bowl, whip the heavy cream with the powdered sugar until soft peaks form. Take a small amount of the whipped cream and fold it into a bowl with the cooled brownie batter to lighten it. Then gently fold in the rest of the whipped cream until no white streaks remain.
Once the brownies are completely cool, spread the chocolate mousse evenly over the top.
Place the brownies in the refrigerator for at least 2 hours to allow the mousse to set properly.
Once set, remove the brownies from the pan. Dust with cocoa powder before slicing into squares.
Notes
Serve the brownies on a beautiful plate, dusting with additional cocoa powder or garnish with chocolate shavings and fresh berries for an elegant touch.