Preheat the oven to 180°C/fan 160°C/gas mark 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 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 3 x 6cm rectangle. Re-roll the trimmings and cut to make more dominoes.
Put on a baking sheet and decorate with the chocolate beans to make the domino numbers. Bake for 8-10 minutes, then allow to cool before serving.