Cream the butter and sugar together in a large bowl. Add the beaten egg and the vanilla extract and stir in. Sift the self-raising flour and coca powder into the bowl and add the chocolate drops. Mix together thoroughly.
Spread onto some cling film and wrap tightly into a cylinder, inside a sheet of silver foil. Chill for at least two hours in the refrigerator.
Preheat your oven to 375 F / 190 C / Gas Mark 5 and line two baking sheets with non-stick baking paper or silicon baking mats. Unwrap the cookie mix and slice into rounds into nearly a centimetre or the thickness of two pound coins. Approximately 12 cookies. Space out on to the trays and place into the oven.
Bake for around eight minutes to ten minutes, until they are firm on the edges and just slightly soft in the very centre if you press them. If you prefer a slightly firmer cookie leave in the oven for a couple of extra minutes.
Remove from the oven and leave on the tray for a couple of minutes to set. Carefully transfer onto a cooling rack and allow to cool.
Melt the white chocolate in a bain-marie and dip in the cookies. Return to baking tray and set aside somewhere cool for the chocolate to set. When the chocolate is set add the holly using some decorative icing or royal icing coloured with a little food colouring. Pipe small green wiggly lines for the holly and red dots for the berries.
