Preheat oven to 350 degrees F (175 degrees C). Lightly grease cookie sheets.
In a medium bowl, cream the butter and sugar.
Add eggs and vanilla; mix until fluffy. Sift together the flour, cocoa, and salt; stir into the creamed mixture alternately with the milk. Drop by teaspoonfuls onto the prepared cookie sheets.
Bake for 8 to 10 minutes.
Meanwhile ... you can make the filling. In a medium bowl, cream the 1/2 cup of butter, 2 cups of confectioners' sugar, vanilla, and salt, until fluffy. When the cookies are cooled, make into sandwiches with the filling and freeze for 1 hour.