To make the cheesecake base, mix the butter, sugar, salt, vanilla seeds, and flour in a bowl and use your fingertips to rub everything together into a rough crumble. Spread the crumble over the base of the tin and press down well, making sure that all areas are covered. Bake on the middle shelf of the preheated oven for around 10 minutes until golden brown. Remove, brush with the egg yolk, and bake for another 5 minutes. Remove from the oven and leave to cool in the tin. Meanwhile, reduce the oven temperature to 150°C (300°F/Gas 2).