To make the peanut buttercream, put the butter and peanut butter into a bowl and beat with an electric mixer. Gradually mix in the icing sugar, and when it starts to thicken, incorporate the coconut milk, 1 tablespoon at a time, until all the sugar is mixed in, and the icing is thick and spreadable. Beat for about 3 minutes until fluffy. Once the brownie is cold, top with a thin layer of the smooth peanut buttercream, return to the fridge to chill.