Cream butter and sugar until light and fluffy. Add sifted flour, and mix well. Shape dough into 1-inch balls and place on cookie sheets. Imprint your thumb in the center to make a 1/2-inch indentation. Fill with your favorite preserves.
Bake 15 to 20 minutes or until golden brown at the edges.
If you use unsalted butter, add a pinch of salt to the cookie dough.