Cream together the butter and sugar until light and creamy. Beat in the vanilla extract and egg, then add the flour, giving you a soft dough consistency. Divide the mixture into two. Beat in a few drops of red food colour into one half, and a few drops of green food colour into the second half. Shape each of the coloured dough balls into a flat rectangle shape and wrap in cling film. Place in the fridge to firm up slightly for 15-30 minutes. Roll each piece of dough on a floured surface to the thickness of a £1 coin, keeping the rectangle shape and both the same size. Sit one on top of the other. Roll up lengthways like a long swiss roll and wrap in cling film. Chill to firm up in the fridge for 30 minutes or longer. Preheat the oven to 180ºC/160ºC fan oven/gas 4. Slice the dough into approximately 1 cm thick pieces and sit each biscuit on a nonstick baking sheet. Generously scatter over sprinkles or silver balls and press lightly to stick. Bake in the oven for 15 minutes then cool on a wire rack.