Preheat oven to 350 degrees. Cream butter in large mixing bowl. Gradually add brown sugar and granulated sugar. Cream well. Add unbeaten egg yolk and vanilla and beat well. Blend in sifted flour to form a stiff dough. Shape dough into small balls. Place on greased cookie sheet. Flatten cookies with bottom of glass dipped in sugar. Bake at 350 degrees for 7-9 minutes, till golden brown (do not overbrown.) Cool before frosting. Garnish with pecan halves. Yields 7 dozen cookies.