Set the oven to 180 C / Gas 4. Lightly butter two sandwich tins.
For the sponge, cream together the butter and sugar until light and fluffy, and then slowly add the beaten egg. Fold in the sifted flour. Spoon the mixture into the prepared cake tins.
Bake for 20 minutes. Leave to cool and then remove from the tins
For the buttercream, beat the butter in a large bowl until soft. Add half of the icing sugar and beat until smooth. Add the remaining icing sugar and one tablespoon of the milk and beat the mixture until creamy and smooth. Add more milk as needed.
To assemble, spread the buttercream on both halves of the cooled cake and put them together.
To decorate, cover the cake in ready to roll icing. With the leftover icing, cut out circles to make snowmen. Dampen the snowmen and stick to the cake, or stick to the cake with a bit of buttercream.
You can use a tube of decorating icing to draw buttons or faces on the snowmen.