Put the flour in a blender with the cinnamon. Add the butter and blitz to form fine breadcrumbs. Add the almonds, sugar, egg yolk and 2-3 tbsp of cold water and blitz to form a soft dough. Knead the dough briefly on a floured surface, then wrap in cling film and chill for 30 minutes.
Meanwhile, chop the marzipan into small pieces. Stir into the mincemeat, along with the sherry.
Preheat the oven to 190C/170C Fan/Gas 5. Thinly roll out the pastry and cut out twelve 7.5cm/3in rounds. Place the rounds in a greased 12 hole tart tin. Divide the mincemeat between the pastry cases.
Cut out 12 stars and place one on top of each tart. Bake for 20 minutes until the pastry is golden and cooked through. Transfer to a wire rack and sprinkle with caster sugar. Serve with cream, custard or brandy butter