Preheat the oven to 180°C, fan 160°C, gas 4. In a large mixing bowl, rub the butter into the flour until it resembles breadcrumbs. Add the cocoa powder and sugar, and mix together. Add the milk (or egg yolk) and mix together. Add a little more flour if the dough is still sticky, and roll into a ball. Chill in the fridge for 20 minutes.
On a floured surface, roll the dough out to 3mm thickness, then trim the edges to make a neat rectangle and cut into long strips that are 3cm wide. Cut the strips into 6cm lengths and score a line (without cutting all the way through) across the middle of each 3x6cm rectangle. Re-roll the trimmings and cut to make more dominoes.
Put on a baking sheet and decorate with the chocolate beans or chips to make the domino numbers – for example, put 2 dots on one half and 3 on the other. Bake for 8-10 minutes. Allow to cool, then play dominoes as you eat!