To make the the pastry, ideally you want to blitz the butter and flour in a food processor until it forms crumbs.
Add the sugar and egg yolk and mix until it forms a dough. Add a dash of chilled milk if you need something to help bind it together.
Take the dough and knead it on a floured surface to form a ball, then wrap it with cling film and chill it in the fridge for half an hour.
Prepare your filling by pitting the cherries and mixing it with the zest and juice of the lemon, the cornflour, sugar and vanilla.
Pre-heat the oven to 180 degrees celsius and take the dough out of the fridge and roll out on a lightly floured surface to 5mm thickness.
Cover the pie dish with the dough and make sure it's pushed into the sides and bottom properly, then add the filling.
Roll out the remaining dough to the design of your choice. You could cover the pie completely, or cut out strips to do a lattice design. Or cut out shapes. If you're covering the pie, make sure to crimp the edges after sealing it with a little brush of water.
Bake for 30 - 35 minutes or until the pastry is golden. Allow it to cool before serving.