1In a large bowl, cream butter and sugars until light and fluffy. Beat in milk and egg. Combine the flour, cocoa, baking powder, baking soda and salt; gradually add to creamed mixture and mix well. Cover and refrigerate for 2 hours or until firm.
On a lightly floured surface, roll out dough to 1/8-in. thickness. Cut with a 1-1/2-in. cookie cutter and place 1 in. apart on greased baking sheets. Bake at 375° for 5-6 minutes or until edges are lightly browned. Remove to wire racks to cool completely.
Combine filling ingredients; spread on bottom of half of the cookies and top with remaining cookies.