Cream together the softened margarine or butter and the sugar. Add the egg yolk reserving the egg white for later use. Add the vanilla flavoring and salt, mixing until all blended. Add the flour and mix until blended. In a separate small dish, beat the egg white with a fork just until frothy and set aside. With the dough, form a small ball (about the size of a marble), roll it in the egg wash, and then roll it in the finely chopped pecans. Place the ball on a greased cookie sheet and press a cherry half on top to flatten out the cookie. Place cookies about 1 inch apart on greased cookie sheet and bake at 350 degrees for 10-12 minutes.