In a bowl, rub the butter into the flour until it resembles fine breadcrumbs. Add the caster sugar, grated chocolate and then a little cold water to bind to a dough. Wrap in cling film and pop in the fridge for 30 minutes.
Preheat the oven to 190 C / Gas 5. Grease a pie dish.
In a saucepan, combine the cherries, demerara sugar, lime zest and juice and nutmeg. Bring to a gentle simmer for 10 minutes.
Remove the pastry from the fridge and roll out, adding a little more water if necessary, to about a 1 cm thickness. Roll enough to line the dish and make a lid.
Gently press the pastry into the prepared pie dish then spoon in the warm filling. Cover with the pastry lid and press together the pastry edges. Trim if necessary and cut two small slits to release the steam.
Bake in the preheated oven for 30 minutes or until the pastry is golden brown and the filling is piping hot. Remove from the oven and serve.