Stir together the butter, sugar, egg yolk and vanilla using a wooden spoon. Stir in the flour - you might need to get your hands in at the end. Tip onto a floured surface and bring together into a smooth dough.
Roll out the dough - half at a time if you like - and stamp out 5-6cm stars. Keep re-rolling trimmings, and you should get about 40 biscuits. Cut out a thin triangle about 2cm long and ½cm wide at one of the indents of each star. Arrange on trays lined with baking parchment, cover with cling film and chill for 30 mins.
Heat oven to 200C/180C fan/ gas 6. Remove cling film and bake the biscuits for 8-12 mins until golden. Cool, then dust with icing sugar. The cooked biscuits can be frozen for up to 3 months.
Mix the coffee and KahlÃºa in a Martini glass. Slowly pour in the double cream over the back of a teaspoon so that it gently settles on the top of the coffee. Dust with a little cocoa and serve with a star biscuit slotted onto the glass and a plateful of Mint chocolate truffles, if you like (see 'Goes well with').