Place the orange peel and sugar in the bowl of a blender and blend until the orange peels are broken tiny pieces. Add the ground almonds, baking powder and almond extract. Blend for a few seconds to mix well.
Beat the egg whites not too firm with a pinch of salt. Then stir them gently to almond mixture with a wooden spoon or spatula. You should have a texture close to home made marzipan.
Form a large roll of dough and wrap it with plastic film. Put in the refrigerator overnight.
D-Day:
Preheat oven to 150°C. Cut the roll into 5mm thick slices then press dough between 2 fingers to get rounded rectangles. Place them not too close to each other on baking tray covered with a baking sheet. Sprinkle with icing sugar.
Bake for 12 to 15 minutes, the cookies should crack but not colour. Sprinkle icing sugar again ice when you take the biscuits out of the oven. Allow to cool 15 minutes on the tray before gently removing the ricciarelli biscuits from the baking sheet.
Cool completely before serving. You can keep the ricciarelli for two weeks in an airtight container but they are best fresh (and then frankly unlikely that remains more than 2 days...).
Recipe by Anne-Sophie Vidal - Fashion Cooking at https://www.fashioncooking.fr/2016/09/ricciarelli-biscuits-siena/