1. In a large bowl, cream butter and brown sugar until light and fluffy. Beat in egg yolk. Gradually add flour and mix well.
2. Shape into 1-in. balls. Dip the tops in sugar. Place sugar side up 2 in. apart on ungreased baking sheets. Flatten with a fork.
3. Bake at 325° for 10-12 minutes or until golden brown. Remove to wire racks to cool.
4. In a small bowl. beat the filling ingredients until smooth. Spread on the bottoms of half of the cookies; top with remaining cookies. Yield: about 3 dozen.