In large bowl, cream butter, (or margarine), with sugar. Beat in egg, ground almonds and extract. Stir flour, baking powder and salt together. Gradually stir into the butter mixture. Shape into a large ball, kneading as you go with a little flour. Chill 1 hour. Preheat oven to 350. Form dough into one inch balls and place 3 inches apart on ungreased cookie sheet. With palm of hand, flatten each cookie and press an almond into the center. Brush with egg yolk. Bake 20-25 min.