110g butter, softened
250g creamy peanut butter
250g icing sugar
3 tablespoons milk, or as needed
Place butter and peanut butter into a bowl and beat with an electric mixer.
Gradually mix in icing sugar.
When it starts to thicken, incorporate milk 1 tablespoon at a time until all the sugar is mixed in and the frosting is thick and spreadable.
Beat until fluffy, about 3 minutes.