Preheat oven to 375 degrees. In a mixing bowl, cream together butter and sugar. Add yolks and vanilla and mix until well blended. Add flour 1/2 cup at a time until soft dough forms. With your hands, roll into 1 inch balls and place 2 inches apart on a cookie sheet. Using your thumb or finger, make an indentation on each cookie ball. Fill each hole with 1/2 teaspoon of desired preserves. Cook 8-10 minutes, until the bottom of cookies are golden brown. Let cool on a wire rack. Dust with powdered sugar, if desired.