Melt the chocolate over low heat or in the microwave (at 650 W stirring every 20 seconds). Set aside to cool.
Weigh 150 g of aquafaba and add it to a large mixing bowl. Whip it with an electric hand mixer for at least 7 minutes.
When the chocolate has cooled down a little, transfer it to a large bowl.
Gently fold the aquafaba foam into the melted chocolate (just like you would incorporate whipped egg whites into a cake batter). Store in the fridge.
When completely cooled, top the brownies with the frosting. Keep the brownies in the fridge. Note: they taste best after resting 24h in the fridge.