Cream butter, sugar, egg and vanilla until fluffy. In a separate bowl, combine flour, cocoa, baking soda and salt. Add to creamed mixture. Shape dough into two 1 1/2" thick rolls. Wrap in waxed paper. Chill for several hours. Place slices on ungreased cookie sheet. Decorate by drawing the tines of a fork across each slice. Bake at 375 F for 8-10 min. or until almost firm.
Meanwhile, combine filling ingredients except food coloring and mint or strawberry extract. Beat until spreading consistency. If desired, divide filling half tint half pink and half green. Add appropriate extract to each one. Cool cookies, then spread wrong side of cookie and top with second cookie.
Makes 36 sandwiches.