Very thinly roll out the pastry on a lightly floured surface. Stamp out 12 x 9cm circles with a fluted cutter, if you have one, and use to line a bun tin. Spread 1 rounded tsp of the cherry jam in the bottom of each tart.
Whizz the cake to crumbs in a food processor, then tip into a mixing bowl and add the flour, butter, ground almonds, caster sugar, eggs and almond extract. Beat together until smooth, then divide between the tarts, spreading a little with the back of a teaspoon to cover the jam. Scatter over the flaked almonds and openfreeze for 2 hrs, then wrap well in cling film and freeze for up to 2 months.
Heat oven to 180C/160C fan/gas 4 with a baking sheet in it. Unwrap the tarts from the freezer and loosely top with a sheet of foil. Bake on the heated baking sheet for 30-40 mins until golden, removing the foil halfway. Cool the tarts a little, then add a splash of water into the icing sugar to make a runny icing. Drizzle over the tarts and leave to set.