My recipe is very similar to yours:
3 cups all-porpose flour
1 cup white sugar
1 cup cold unsalted butter cut into cubes
1/2 tsp salt
4 large egg yolks
1tsp vanilla essence or 1 tsp grated lemon zest
1 egg white beaten with 2 tsp water
Place the sugar, flour, butter and salt in the bowl of your food processor. Whiz until the mixture resembles fine breadcrumbs.
In a small bowl, lightly beat the egg yolks and lemon zest, then, with the motor running, add this to the dry ingredients and process until the dough forms, then remove from the machine.
Roll the dough by hand to form a long sausage, about 2 inches in diameter, or a little bit less. Then roll it in food wrap and either: freeze until required or: put into the fridge for an hour to rest and cool.
To bake: cut the sausage of dough into 1/2 inch rounds, place on baking tray, brush with beaten egg white and sprinkle with sugar. Cook in the oven for about 25 minutes at 180° Celsius. When cooked, place the biscuits to cool on a cake rack, then store in a tin with a tight-fitting lid.
Enough is never as good as a feast Oscar Wilde