Preheat the oven to 160C fan and line a cupcake tray with 12 cases.
Beat together the butter and sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition.
Sift in the flour, cocoa powder, and baking powder, then mix until combined.
Add milk gradually until the batter is smooth.
Divide evenly between cupcake cases and bake for 20-22 minutes.
Let cupcakes cool completely on a wire rack.
For the frosting, beat the butter until creamy, then add icing sugar, Nutella, and milk. Whip until fluffy.
Pipe the frosting onto cooled cupcakes and top each with a piece of Kinder Bueno.