Jam Filled Butter Cookies

¾ cup butter, softened
¾ cup margarine, softened
1 cup sugar
4 egg yolks
1 tsp. vanilla (or almond flavoring)
3 ½ cups flour

1 cup fruit preserves, any flavorDirections Preheat the oven to 375 degrees F (190 degrees C). In a medium bowl, cream together the butter, white sugar, vanilla and egg yolks. Mix in flour a little bit at a time until a soft dough forms. Roll dough into 1 inch balls. If dough is too soft, refrigerate for 15 to 20 minutes. Place balls 2 inches apart onto ungreased cookie sheets. Use your finger or an instrument of similar size to make a well in the center of each cookie. Fill the hole with ½ teaspoon of preserves. Bake for 8 to 10 minutes in the preheated oven, until golden brown on the bottom. Remove from cookie sheets to cool on wire racks.