To start, make the shortcrust pastry base. In a large bowl mix together the butter and sugar with your hands, and finally add the egg yolks and mix. Add the flour and beat until a soft dough is formed. Gather the dough together and press it into the bottom of the tin and 2cm up the sides. Bake for about 18-20 minutes, or until golden. Remove from the oven, but leave the oven on whilst you make the filling.