Cream butter with extract; add sugar gradually, beating until fluffy. Add egg yolk and beat thoroughly. Sift together flour, baking powder and salt; add to creamed mixture in fourths, mixing until blended after each addition. Roll dough into 1 inch balls. Make impression with thumb. Fill with 1/4 teaspoon jelly or jam before baking. Bake at 350 degrees about 12 minutes.