Preheat the oven to 190˚C (170˚C fan oven, 375˚F, gas 5). Line baking trays with baking parchment. In a mixing bowl, beat together the butter and sugar until creamy. Beat in the egg yolk.
Sift the flour, Cocoa and Baking Powder on top and add 75g (3oz) of the Chocolate Chips. Mix together to form a firmish dough.
Divide the mixture into 12 and form into balls. Place a little apart, on the prepared baking trays; press down gently to flatten them into rounds. Push the remaining Chocolate Chips into the cookie dough, flat side up, to resemble the fruit in a Christmas pudding. Bake for 15-18 minutes until just firm. Cool for 5 minutes then transfer to a wire rack.
To decorate, knead the Ready to Roll Icing to soften it. Roll out the Green Icing thinly on a lightly dusted work surface, and cut out 24 x 2cm (3/4 inch) holly leaf shapes using a small cutter, re-rolling the icing as necessary. Form the Red Icing into tiny balls for the holly berries.
Line a board or tray with baking parchment. Lay the cookies on the parchment. Melt 1 packet of the white cooking chocolate in a microwave proof bowl either over a pan of boiling water, or in a microwave in 30 second blasts, stirring in between (be careful not to overheat the chocolate). Using a teaspoon, thickly spoon the chocolate over the tops of the cookies, allowing it to drip a little.
Finally decorate with the holly leaves and berries before the white chocolate sets. Leave for a few minutes to dry in a cool place before serving.
These festive treats make lovely gifts. Pop in a cellophane bag and tie tightly with festive ribbon. Will store like this for up to 2 weeks.
For an extra festive flavour, add 1 tsp ground cinnamon or ground mixed spice to the cookie dough, and replace the caster sugar with dark soft brown sugar.