Place the butter, sugar, egg yolks, vanilla extract and salt in the bowl of a food processor. Process until the butter is creamed.
Add the flour and pulse until it just comes together. Use a spatula to push down the sides. Give it a few more pulses. Take out the blade. Tip the dough out on to a work surface and bring it together into a soft dough. Form it into a roll approx 2 inches in diameter.
Lay a sheet of cling film on a work surface. Place your roll of dough on the cling film and wrap it up, twisting the ends closed. Put in the freezer or fridge to harden.
Take out, remove from the cling film. Using a sharp knife slice into discs about 1.5cm thick. Place onto a lined baking tray and cook in an oven preheated to 180C for 25 to 30 minutes.