icing sugar, proper butter, bit of food colouring (if required). whizz it up, wap it in a piping bag, take the middle out of your microwave and stick the cupcake on that so you can turn it evenly whilst you pipe it on and bob's your uncle!
Good tip re the microwave! If you want white icing, you must use Lurpak.
Oh and make sure its nice and soft. I've never quite got the ratio of sugar/butter just right, if you do it too soft (ie not enough sugar) it can look a bit sloppy. I try to practise on a plate first, then scrape it all back into the bowl.
for enough to generously decorate 12 to 15 cupcakes with vanilla buttercream you will need 250g butter, 500g icing sugar, 2 tbsp milk, 1tsp vanilla extract. Leave the butter out of the fridge for at least an hour before you start to allow it to soften a little. Use an electric whisk. Whisk the butter in a bowl until it's really smooth and soft and the colour starts to lighten, then add the milk and vanilla extract and whisk again until fully combined and the mixture is smooth again, then gradually add the icing sugar using a spoon first to avoid an icing sugar cloud covering your kitchen, then whisk for a few minutes till light and creamy. To decorate you will need a piping bag and nozzle (the microwave plate thing is a good idea) start from the outside edge of the cake and swirl your way in to the middle