Beat sugar and butter until light. Add the egg, then the sifted flour, and mix well. Let dough rest 2 hours in a warm place in a bowl covered with a clean towel.
Roll as thin as possible on a floured board. Cut with a floured round cookie cutter. Put a small amount of almond filling in the center of each cookie, and fold the dough over triangularly to make a hat shaped as much as possible like Napoleon's.
Place cookies on a buttered and lightly floured cookie sheet. Bake 18 to 20 minutes in a preheated 350° oven. Watch cookies carefully to make sure the are not getting burned.