Preheat oven to 350°. In a bowl, cream butter and sugar. 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 the creamed mixture, beating until well combined. , Drop by tablespoonfuls 2 in. apart onto ungreased baking sheets. Bake at 350° for 8-9 minutes or until puffy and tops are cracked. Invert half of the cookies onto wire racks. Immediately place a mint patty on each, then top with remaining cookies. Press lightly to seal. Cool completely.