100g raspberry jam (or use lemon curd or chocolate spread)
140g icing sugar, sifted
food colourings (optional)
Heat oven to 180C/160C fan/gas 4 and line 2 large baking sheets with baking parchment. Put the butter, sugar, egg yolk, vanilla and ¼ tsp salt into a large bowl and beat with a wooden spoon until smooth. Stir in the flour to make a stiff dough, then shape into 2 discs, cover with cling film and chill for 10 mins.
With plenty of extra flour on the work surface and rolling pin, roll a batch of dough to the thickness of a £1 coin. Stamp out as many 7cm rounds as you can. In half of the rounds, stamp out 2 eye sockets with an apple corer, then use well-floured fingers to pinch the jaw shapes. Lift onto the baking sheets. Bake for 15 mins or until pale golden, then cool for 2 mins before lifting onto a wire rack.
Sandwich the cold biscuits together with the jam. Mix the icing sugar with 4 tsp water until smooth, divide into 3, then colour as you like. Put into piping bags, snip off the tips, then pipe spooky patterns and faces onto the skulls.