Mix the butter and caster sugar together in a large bowl with a wooden spoon, then add the egg yolk and vanilla extract and beat to combine. Stir in the flour and chocolate drops. At this point you may need to use your hands towards the end to give everything a good mix and pull the dough together.

Divide the dough in two and roll each piece into a log about four centimetres wide.

Roll this in parchment paper and seal the ends by twisting the paper and tying with a piece of ribbon or raffia. Label with instructions to cut into slices and bake for eight to 10 minutes at 180 degrees/gas 4.