Whisk egg and granulated sugar until light and fluffy. Sift flour, salt, cinnamon, white pepper, Mixed Spice, cornstarch and baking powder into egg mixture. Add lemon peel and candied peel and stir well. Chill 1 hour. Preheat oven to 350'F. (175'C.). Lightly grease several baking sheets. Form mixture into 16 small balls and place well apart on baking sheets. Bake in preheated oven 20 minutes or until well risen and lightly golden. Cool on a wire rack. Sprinkle with powdered sugar, if desired. Store several days before serving to allow time for flavors to mellow.