After sifting the flour thoroughly, mix with salt and cold butter cubes. Work the mixture first with hands to spread the butter evenly until the dough resembles thick crumbs. Add the ground almonds, chopped in blender, together with the traces, the zest and cinnamon. Add the egg and mix well. Cover the bowl with a cloth and let sit in the refrigerator for an hour. Wash the plums thoroughly and remove the seeds. Cut the fruit into thin slices or chunks. Remove the dough from the refrigerator. Lightly butter a round cake moud (without the hole in the middle) or a pyrex. Pour the dough into the mould, top with plum slices and press down gently with fingers.