Crumble the cake into fine crumbs using your hands or in a food processor. Put into a bowl. In another bowl, beat together the butter and icing sugar until creamy.
Melt the plain chocolate in the microwave or in a bowl over a pan of barely simmering water. Add the melted chocolate to the butter mixture and combine well. Add this to the cake crumbs and mix thoroughly using your hands, until it sticks together. Divide the mixture into eight and shape each one into a smooth ball. Insert a cake pop stick, place on a tray lined with baking parchment and chill for at least 2 hours.
Melt the milk chocolate in the microwave or in a bowl over a pan of barely simmering water. Take a cake pop and dip in the chocolate. Shake gently until the excess has drained off. Push into a polystyrene block.
Make two holes in the top of the head and insert a pretzel 'antler' in each one. Press a sweetie nose onto the front and hold for a few seconds until attached. Repeat with all the other cake pops. Leave to set for about 20 mins and then using black icing, pipe on eyes and a mouth.