In a large bowl, chop the shortbread biscuits into small pieces, taking care not to crumble them.
Melt the butter in the saucepan, add milk, sugar, cocoa and eggs and cook for a few minutes, stirring constantly. Remove the saucepan from the hob, stir in the vanilla sugar and pour everything over the crushed biscuits. Stir well and place in a sausage tin on aluminum foil, cling film or in a freezer bag.