Place 16 mini silicone muffin cases on a baking sheet.
Put the flour and butter in a food processor and blend until the mixture resembles fine breadcrumbs, then lightly blend in the sugar. Add the egg yolk and vanilla extract and blend to make a soft dough. Shape the dough into 16 small balls and press a piece into the centre of each case.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 12–15 minutes until just starting to colour around the edges. Leave in the cases for 10 minutes, then transfer to a wire rack to cool.
Melt the chocolate (see