Chocolate Mint Whoopie Pies

In a large bowl, beat sugar and oil until crumbly. Add egg; beat for 1 minute. Combine the flour, cocoa, baking soda and salt. Gradually beat into sugar mixture. Add milk and mix well. (Dough will be sticky.) With lightly floured hands, roll dough into 3/4-in. balls., Place 2 in. apart on baking sheets coated with cooking spray. , Flatten slightly with a glass coated with cooking spray. Bake at 400° for 4-5 minutes or until edges are set and tops are cracked. Cool for 2 minutes before removing to wire racks to cool., In a small bowl, combine butter and confectioners' sugar until crumbly. Beat in extract, food coloring if desired and milk. Spread on the bottoms of half of the cookies; top with remaining cookies.