Preheat the oven to gas 4, 180°C, fan 160°C. Line a cupcake tin with 12 cases.
Put the butter and sugar in a bowl and beat until light and fluffy. Gradually beat in the eggs and vanilla until smooth, adding a little flour if the mix curdles. Gently fold in the flour.
Divide the mix equally between the cupcake cases and bake for 15-18 minutes until firm and golden. Cool.
To make the buttercream icing, place the butter in a bowl with 1-2tsp of boiling water. Add the icing sugar and beat until smooth. Place in a piping bag with a large star nozzle and pipe the buttercream onto the cakes. Decorate with silver balls. The cakes will keep for up to a week in a cake tin.
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