
Put the flour, butter and 85g of the icing sugar, vanilla and egg yolks in a food processor and pulse in burst until combined. Add 1/2 tablespoon water if the mix feels a little dry. Tip the rough mixture onto a work surface and briefly knead until the dough has come together. Wrap and chill for 30 mins.
Heat the oven to 180C/160C fan/gas 4. Line two baking sheets with baking parchment. Lightly dust your work surface with flour and roll the dough out to around 3mm thick. cut 24 rounds from the dough with a 7cm cutter. Bring the remaining scraps of dough together and reroll to make more biscuits. Line up the discs of dough on the baking sheets, then bake for 10-12 mins or until lightly golden brown around the edges. Transfer to a wire rack and leave to cool completely.
the remaining icing sugar with 2-3 teaspoons of water to make a very thick icing. Spoon or pipe the icing over the top of half of the biscuits, leaving a border around the edge of each one. Leave to set for 30 minutes. Spread the jam over the un-iced biscuits and sandwich together with the iced halves.