Mix the softened butter with the flour until you get crumbs. Add the vanilla, sugar and egg yolks and knead until homogeneous.
Cover the dough and leave it in the fridge for about 30 minutes. Make small balls, flatten them and put a piece of chocolate in the middle.
Bake in a baking dish with baking paper in a preheated 392°F (200 °C) oven for about 10 minutes.
Once cooled, decorate.