Preheat oven to 350.
Line a 9x13-inch baking pan with foil and grease lightly with cooking spray; set aside.
In a large mixing bowl, whisk together the sugar, brown sugar, melted butter, milk, eggs, and vanilla.
In a separate mixing bowl, combine flour, cocoa, baking powder, and salt.
Fold the dry ingredients into the wet until combined.
Pour the batter into prepared pan.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out with few moist crumbs.
Place the pan onto a cooling rack and let cool completely
In the meantime, prepare the chocolate mousse frosting.
Place all ingredients in a mixing bowl and whip until mixture holds a stiff peak.
Remove the cooled brownies from the pan by holding onto the sides of the foil.
Spread the chocolate mousse frosting over the entire cake; cut into squares, garnish with grated chocolate, and serve.