Pastry: Preheat oven to 375 degrees F. Line baking sheets with parchment paper and set aside. To make the whoopie pies, combine flour, cocoa powder, baking soda and salt in a medium bowl and set aside. In a large mixing bowl, beat butter on medium or high for 30 seconds. Add brown sugar and beat until light and fluffy. Add in egg and vanilla and mix well. Alternately add buttermilk and flour mixture, 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 space between rounds. Bake until tops are set, 7 to 9 minutes for 1 inch cookies or 9 to 11 minutes for 2 inch cookies. Cool completely on baking sheets on rack and then remove cookies from baking sheets.
Filling: In a medium mixing bowl beat the butter and cream cheese until smooth. Fold in the jar of marshmallow creme. Spread the filling on the flat side of half of the cookies and top with the remaining cookies, flat side down. Store refrigerated in an airtight container.