Preheat the oven to 180°C/350°F/gas mark 4. Arrange paper cases in A 12 case cupcake tin.
Beat the butter and caster sugar together until smooth. Add the egg gradually making sure to mix the mixture well.
Then spoon the mix into the paper cases. Bake for 20 minutes until golden brown. To ensure the cupcakes are cooked stick a metal skewer into one an if it is clean then take them out if not leave for another 3 minutes and remove. Cool on a wire rack.
While the cupcakes are cooling make a start on the frosting.
Beat the butter until soft and smooth. Then begin to beat in the double cream. Sift in icing sugar to the mix to make sure there are no lumps. Then beat mixture until smooth. Then add the peanut butter and mix well.
Spoon the frosting into a piping bag and begin to decorate your cupcakes giving each one a generous swirl on top.
You can chop up Reeses Peanut Butter Cups to decorate or simple sprinkle some grated chocolate over the top.