Preheat oven to 375 degrees F. Line a cookie sheet with parchment paper; set aside. In a large bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add brown sugar, cocoa powder, baking soda, and salt. Beat until combined, scraping sides of bowl occasionally. Beat in egg and vanilla until combined. Alternately add flour and buttermilk, beating on low speed after each addition just until combined. Stir in food coloring.
Drop dough by rounded teaspoons 2 inches apart onto prepared cookie sheet. Bake for 7 to 9 minutes or until edges are set. Cool on cookie sheet for 2 minutes. Transfer cookies to a wire rack; cool.
Spread Peppermint and Cream Cheese Filling onto bottoms of half of the cookies. Top with remaining cookies, bottom sides down. If desired, sprinkle with peppermint candies before serving.
Frosting
In a large bowl combine cream cheese, butter, and peppermint extract. Beat with an electric mixer on medium speed until light and fluffy. Gradually beat in powdered sugar. If necessary, add milk (1 teaspoon at a time) to make a filling of spreading consistency.