For the cookies: In a large bowl beat the butter for a minute until it's smooth and creamy. Then add both the sugars and using a hand-held mixer, mix on a medium speed until light and fluffy. Add the egg and vanilla, and beat on a high speed until thoroughly combined.
In a separate bowl whisk together the flour, cocoa powder, baking powder and salt. Then on a low speed, slowly mix into the wet ingredients until combined. The cookie dough will be quite thick at this point. Switch to a high speed, and add in the milk and dark chocolate chips. The cookie dough will be very sticky and look hard to handle at this point, so it must be chilled before using.
Cover with clingfilm and place in the fridge for at least 3 hours, and up to three days. (I chilled mine overnight).
Remove the cookie dough from the fridge and leave to sit at room temperature for 20 minutes, or 30 minutes if you chilled for longer than 3 hours. Preheat oven to 175C and line two large baking trays with baking paper. Scoop and roll balls of cookie dough and place onto the baking trays, I allowed for about 2tbsp of dough per cookie.
Bake the cookies for 10-12 minutes. They will feel very soft when you remove them from the oven, so leave to cool on the baking tray for 10 minutes. Then place on a cooling rack to cool completely.
For the decoration: Melt the white chocolate over a bain marie being careful not to burn the chocolate. Leave to cool for at least 10-15 minutes, then place into a piping bag. Snip off the end, to make a very small hole and pipe the outline of the christmas pudding decoration to make a dam. Then fill the outlines with the remaining chocolate, using a toothpick to smooth out and push the chocolate into the edges. Add the holly and berry decorations and leave to set at room temperature.