Preheat the oven to 200°C/gas mark 6. Roll out half the pastry on a surface lightly dusted with icing sugar to about 3mm and use to line a 20-22cm round pie dish.
Drain off any excess liquid from the defrosted cherries and discard. Add the cherries to a medium bowl along with the sugar, cornflour, lemon juice and almond extract. Stir to combine.
Combine the egg yolk with 1 tbsp cold water and brush the base and sides of the pastry. Sprinkle 1 tbsp demerera sugar on to the base and sides of the pastry. Using a slotted spoon, spread the cherry mixture over the bottom of the lined tin.
Roll out the remaining pastry and stamp out pastry hearts using a 3cm heart-shaped cutter. Place the hearts on top of the cherry filling, overlapping each slightly, until the top is completely covered. Brush all over with the eggwash and sprinkle with the remaining demerara sugar.
Bake in the oven for 40-45 minutes until the pastry is golden. Remove from the oven and cool on a rack for at least 5 minutes before serving.