Preheat oven to 375 degrees F. Line baking sheets with parchment; set aside. In medium bowl combine flour, cocoa powder, baking soda, and salt; set aside.
In large mixing bowl beat butter on medium to high 30 seconds. Beat in brown sugar until light and fluffy. Beat in egg and vanilla. Alternately add flour mixture and buttermilk, beating after each addition just until combined. Stir in food coloring.
Spoon batter in 1- or 2-inch diameter rounds, about 1/2-inch high on prepared baking sheets, allowing 1 inch between each round.
Bake 7 to 9 minutes for 1-inch cookies or 9 to 11 minutes for 2-inch cookies, or until tops are set. Cool completely on baking sheets on rack. Remove cooled cookies from baking sheets and frost with favorite filling.
For the Filling:
In the bowl of an electric mixer fitted with a paddle attachment, beat the butter until smooth with no visible lumps. Add the cream cheese and beat until combined. Add the powdered sugar and vanilla and beat until smooth. Be careful no to overbeat the filling, or it will lose structure.
Pipe filling in between onto bottom of one whoopie pie and sandwich between another whoopie pie. Store whoopie pies in fridge until ready to serve, then allow whoopie pies to sit at room temperature for 10 minutes before serving.