1. In a large bowl, with a mixer on medium speed, beat 1 cup butter and sugar until smooth. Add egg yolk, liqueur, and orange peel and beat until well blended.
2. In another bowl, mix 2 cups flour, baking powder, and salt. Add to butter mixture; stir to mix, then beat until well blended. Gather dough into a ball, divide in half, and flatten each portion into a disk; wrap each tightly in plastic wrap and freeze until firm enough to roll without sticking, about 30 minutes.
3. Unwrap dough. On a lightly floured surface, with a floured rolling pin, roll one disk at a time to about 1/4 inch thick. With a floured, 2-inch round cutter, cut out cookies. Place about 2 inches apart on buttered 12- by 15-inch baking sheets. Gather excess dough into a ball, reroll, and cut out remaining cookies.
4.In a small bowl, beat egg white with 1 teaspoon water to blend. Brush cookies with mixture and sprinkle or arrange about 1/2 teaspoon sliced almonds on each.
5. Bake cookies in a 325º regular or convection oven until lightly browned, about 15 minutes; if baking two sheets at once in one oven, switch their positions halfway through cooking. Let cookies cool on sheets for 5 minutes, then use a wide spatula to transfer to racks to cool completely.