In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine the flour, cocoa, baking soda and salt; gradually add to creamed mixture alternately with milk, beating well after each addition. Refrigerate for at least 2 hours.
Drop by rounded teaspoonfuls 2 in. apart onto greased baking sheets. Bake at 375° for 10-12 minutes or until edges are set. Remove to wire racks to cool.
In a small bowl, combine filling ingredients ; beat until smooth. Spread on the bottoms of half of the cookies; top with remaining cookies. Store in the refrigerator.
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