To make the biscuits, preheat oven to 200C/180C Fan/Gas 6.
Beat the butter and sugar in a bowl until pale and creamy. Gradually beat in the egg yolks until fully incorporated, then beat in the rest of the ingredients.
Roll out the dough on a lightly floured surface until about 5mm/¼in thick. Cut out the biscuits using a 6cm/2in cutter and arrange them on a non-stick baking sheet, re-rolling and cutting any trimmings. Transfer the biscuits to the fridge until firm.
To make the citrus sugar, mix the zests with the sugar in a small bowl until combined.
Bake the biscuits for 8–10 minutes, or until they are starting to turn a pale golden colour. Sprinkle the hot biscuits with the citrus sugar and leave to cool on a wire rack.
To make the fruit medley, blend the raspberries with the sugar and lemon juice in a food processor, then pass through a sieve.
Divide the purée between four serving glasses, top with a mixture of the fruits and decorate with mint leaves.
In front of the guests, crack open the Champagne and pour over the top of the dish. Serve with the citrus-spiced biscuits.