Preheat the oven to 220C/425F/gas mark 7.
To make the filling, put the ground almonds, sugar and jam in a bowl with a pinch of salt and mix well. Add the egg white and stir until you have a thick paste, rather like marzipan.
Lay out the puff pastry sheets on a work surface and cut out 5 triangles from each one, 12-14cm wide at the base and 24cm long on the sides. Spoon 1 tsp of almond paste onto the base of each triangle, centred and about two-fingers’ width away from the edge. Resist the urge to overfill here: you really just need 1 tsp, or the filling will spill out in the oven.
Fold the bottom edge of the triangle over the filling, trying to tuck it under, and roll the pastry up as tightly as you can. Gently fold the tips under into the horns of a croissant, pinching where needed to seal. Repeat this with the remaining pastry triangles, then arrange them on a baking sheet lined with baking parchment
Brush the croissants with the beaten egg yolk to glaze, then sprinkle liberally with the fl aked almonds. Bake for 20 minutes, until golden all over. Remove the tray from the oven and sift the icing sugar generously over the piping-hot croissants, so that some of it melts into them. Let them cool slightly, then transfer to a wire rack. Eat warm or cold.