Pre-heat your oven to 160C Fan/350F/Gas Mark 4 and line a cupcake tin with cupcake cases
For the cupcake sponge, mix together the butter or baking spread and caster sugar, ideally using an electric mixer, until smooth and fluffy
Add the eggs, and mix until fully incorporated
Add the self raising flour, cocoa powder and baking powder, and whisk or fold in gently
Divide the mixture between the cupcake cases, and bake them for 20-25 minutes or until a thin skewer inserted in the centre of the cupcakes comes out clean. Put them on a rack to cool completely
To make the buttercream mix the butter on it's own until it's smooth and creamy, for 2-3 minutes. Use an electric whisk or stand mixer with the paddle attachment for best results. Then mix in the icing sugar, Nutella and milk. Mix until smooth. If the buttercream is too stiff then you can add a little more milk
Pipe or spread the buttercream onto the cupcakes. Decorate with Kinder Bueno pieces and chocolate strand sprinkles, or other decoration of your choice
Store in an airtight container in a cool place, eat leftovers within 3 days