Preheat the oven to 200°C / 390°F (180°C fan). Remove one of the pastry discs from the fridge and roll out on a lightly floured surface (you can also use cocoa powder instead of flour if you like). You want the pastry to be big enough to fill a 9 inch pie dish (mine is about 2 inches deep).
Carefully place the pastry into the pie dish, tucking it into all the edges with your fingers. Crimp the edges of the pastry with your fingers to help stop the pastry from shrinking in the oven. Don’t stretch the pastry, as otherwise it will un-stretch again in the oven!
Spoon the chocolate cherry filling into the pastry case. Try to avoid adding in the extra juices at the bottom of the bowl – just pour these away. Sprinkle the top of the cherries with the chocolate chips, and then gently mix together (without damaging the pastry!)
Take the second disc of pastry out of the fridge and roll out as before. Cut out shapes or strips of pastry to decorate the top of the pie. I went for a lattice effect. Once you have covered your pie, brush the pastry with beaten egg to help give it that nice sheen once baked.
Bake the pie for 20 minutes at the current temperature, and then reduce to 190°C / 375°F (170°C fan) for a further 30 to 35 minutes. After the first 20 minutes, I also like to very gently cover the top of the pie with aluminium foil to stop the pastry browning too quickly. I also like to tear a little hole in the middle of the foil to let the steam out. Or you can use a pie shield if you have one.
Once the pie has finished baking, leave to cool on a wire rack for at least an hour to let the filling set. Either serve warm immediately with ice cream, or leave to cool completely, cover with cling film or plastic wrap, and store in the fridge for a day or two. The pie can be reheated at 180°C / 350°F (160°C fan) for 20 minutes or so if wanted.