Rub the butter into the flour to make crumbs, then stir in sugar, vanilla, yolks and chocolate chips and bring together to form a dough. Roll out on a lightly floured surface and stamp out 6cm round biscuits. Re-roll the trimmings and repeat. Arrange on parchment-lined trays and chill for 30 mins.
Heat oven to 200C/180C fan/gas 6. Bake biscuits for 10-12 mins until golden. Cool. Spread each with white chocolate and leave to set. Decorate, or write on buyer’s name with red icing on the spot, if you like.