First, make the pastry. Put the butter and flour in a food processor and whizz until the mixture looks like damp breadcrumbs. Add the sugar and pulse to combine. Whisk the egg yolks, 2 tbsp cold water and the vanilla in a bowl, then drizzle over the flour mixture. Pulse until the dough starts to clump together, adding a drizzle more water, 1 tsp at a time, if the dough feels dry – try not to add too much or the pastry will be tough. Tip out onto a work surface and knead briefly to bring everything together to a smooth dough. Break into 2 pieces, one a little larger than the other, wrap in cling film, pat into flat discs and chill for 30 mins.