Cream butter and sugar until light and fluffy. Add egg and egg yolk. Add almond and vanilla extracts; beat well.
In a separate bowl, sift together the flour, baking powder, and salt. Mix ground almonds into the flour mixture. Add flour and nut mixture to the creamed mixture; stir to blend well.
Divide dough into two portions. Shape each portion into a roll about 2 inches in diameter. Wrap rolls in waxed paper, twisting ends. Refrigerate for about 1 hour, or until thoroughly chilled.
When ready to bake, slice dough into 1/4-inch thick rounds. Cut slices in half and shape the flat edge of the cookie with your fingers to form a curve, so the cookie looks like a crescent. Place crescents on lightly greased baking sheets and bake at 350° for 10 minutes. Remove almond cookies to racks and sprinkle with sifted powdered sugar while still hot.
Makes about 3 dozen cookies.