100g unsalted butter, roughly chopped
100g golden caster sugar
1 tsp vanilla extract
150g plain flour, plus extra for dusting
1 tbsp cocoa powder
1 large egg yolk
100g icing sugar
1. Put the butter, sugar and vanilla in a food processor and whizz until smooth. Add the flour, cocoa and egg yolk and whizz again until the mixture comes together in a ball. Turn out onto a large sheet of baking paper and briefly knead the dough, until smooth. Wrap in baking paper and chill for 30 minutes.
2. Preheat the oven to 180C/ fan160C/gas 4. Dust the cookie dough with plenty of flour and roll out between 2 sheets of baking paper to a thickness of 2mm. Remove the top layer of paper. Using a heart-shaped cookie cutter (about 7cm), cut out 15 hearts from the dough. Re-roll the pastry trimmings between 2 sheets of baking paper, then cut out 3-5 more hearts. Place the hearts, spaced well apart, on baking paper and slide onto 2 large baking sheets. Use the narrow end of a small plain piping nozzle to stamp holes in the top centre of each heart, about 3mm wide. Bake for 12-15 minutes until the cookies are firm and golden. Cool for 5 minutes, then transfer the cookies to a wire rack to cool completely.
3. Sift the icing sugar into a bowl, gradually add 2 teaspoons of cold water, stir, then add another 1 teaspoon of water to make a smooth consistency.
4. Put a fine plain nozzle into a plastic piping bag and fill the bag with the icing. Pipe various designs around the edges of each cookie, then set aside to let the icing firm up.
5. Thread some white raffia through the holes in each cookie and tie a knot. Hang the cookies onto twigs to display and serve. Or, to make a gift, wrap in cellophane and tie with red ribbon.