For the pastry, dice the butter, then place in a food processor with the flour and icing sugar. Blitz until the mixture resembles breadcrumbs.
Add the egg yolks, one at a time, then gradually add 1 to 2 tablespoons of chilled water until the mixture forms a ball.
Knead on a lightly floured surface for 30 seconds. Divide in half, wrap each half in clingfilm, then refrigerate for at least 1 hour.
Lightly grease a 22cm (across the top) round metal or glass pie dish with butter.
Remove the dough from the fridge and roll out one piece on a lightly floured surface into a 30cm circle, roughly 4mm to 7mm thick.
Drape the dough over the rolling pin, then carefully unroll it over the dish, gently pressing it into the bottom and sides. Cut off the excess pastry, leaving a 2.5cm overhang around the edge of the pie dish. Chill the pastry case while you prepare the filling.
For the filling, destone the cherries, then place in a bowl with the lemon zest and juice, flour, cornflour, sugar and vanilla extract. Mix well.
Leave to stand uncovered, at room temperature, for 10 minutes. Remove the pie dish from the fridge and spoon the filling evenly into the pastry shell. Dot the top with diced butter.
Preheat oven to 200ºC/400ºF/gas 6.
Roll out the remaining pastry as before. Beat the egg with the milk, then brush it around the edge of the pastry. Drape it over the rolling pin and place it over the pie. Trim off the excess from the rim and press the edges firmly together to seal.
Cut 3 slashes on top, brush with the egg wash and bake for 10 minutes.
Lower the heat to 180ºC/gas 4 and bake for a further 40 minutes or until golden. Allow to cool before serving.