Preheat the oven to gas 4, 180°C, fan 160°C. Put the butter into a blender, add the flour and then pulse until the mixture resembles fine breadcrumbs.
Stir in the cocoa powder, the sugar and a pinch of salt and then the egg and pulse until just combined. Add more flour if the mixture is still sticky, then tip out the dough onto a floured work surface and roll into a ball.
Roll out the dough to the thickness of a pound coin, then trim the edges to make a rectangle and cut into long, 3cm (1¼in)-wide strips. Then cut the strips into 6cm (2½in) lengths and score a line (don’t cut all the way through) across the middle of each one. Make more from the trimmings.
Decorate with mini Smarties to make the domino spots (once baked, the colours will fade, so use the brightest colours for the best results). Bake for 15 minutes or until just firm to touch. Leave to cool on the baking tray for 5 minutes and then transfer to a wire rack to cool. Once cool, pack into paper cups.
Freezing and defrosting guidelines
Bake according to the recipe but stop before adding decoration or icing if called for. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months.
Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving. If you wish to serve the baked goods warm, place them on a baking sheet for 10 minutes in a moderate oven.
If you wish to add icing or decoration, add them after the baked goods have fully defrosted and are at room temperature.