1. Heat oven to 350 degrees . Have 2 whoopie pie pans ready. In a medium bowl, whisk flour, cocoa powder, baking soda and salt.
2. In a large bowl, beat butter with an electric mixer for 30 seconds, until creamy. Beat in brown sugar for 1 minute, until fluffy, and beat in egg for 1 minute. Beat in vanilla. On low speed, beat in flour mixture, alternating with buttermilk, beginning and ending with flour. Divide evenly between pans, spreading batter slightly until level.
3. Bake at 350 degrees for 11 to 13 minutes or until dry to the touch. Remove from pans to a wire rack to cool.Filling
4. In a medium bowl, beat butter and cream cheese until smooth. Fold in marshmallow creme and tint with food coloring to desired color. Spread 1 to 2 tbsp of the filling onto flat side of a cookie. Sandwich with a second cookie. Repeat; refrigerate pies in an airtight container up to 4 days. Let stand at room temperature before serving.