Preheat the oven to 350F/180C and whisk together the flour, cocoa, baking soda and salt.
In a large bowl, cream the butter and sugar together until light and fluffy.
Beat in the cream cheese with the egg yolk and vanilla extract until completely combined.
Use a spoon or spatula to stir in the dry ingredients you mixed together in the first step.
Mix in the milk chocolate chunks and refrigerate the dough for about 15 minutes until it's less sticky and easier to roll into balls.
When the dough's ready, roll it into around 22 ping pong ball-sized balls and place them around 2 inches apart on a baking sheet. Don't squish them together - if you have to bake in two lots, that's fine!
Bake for around 14-18 minutes, or until the cookies are fully set. Leave them on a wire rack for several hours to cool completely.
When cooled, divide the white chocolate into two bowls and melt in the microwave or double broiler.
Add a few drops of food colouring to each bowl and stir until you've got your desired Christmas colour. If the chocolate is a bit thick, add some vegetable oil to it ½ teaspoon at a time and stir until it thins out a bit.
Scoop each coloured chocolate into a piping bag and pipe on top of the cookies. If you can't be bothered with the hassle, skip the piping bags and just drizzle it on with a spoon.