1. For the Cookies, preheat oven to 350°F. Mix flour, cocoa powder, baking soda, & salt in medium bowl; set aside. Mix buttermilk and vanilla in small bowl.
2. Beat butter and brown sugar in large bowl with electric mixer on medium-high speed until light and fluffy. Add egg; mix well. Add flour mixture alternately with buttermilk mixture, beating on low speed after each addition until smooth and scraping down sides of bowl occasionally. Spoon 1 tablespoon of batter, 2 inches apart, onto parchment paper-lined large baking sheets. (Cookies will spread so avoid crowding them on baking sheet.)
3. Bake 8 minutes or until cookies are puffed and spring back when touched, turning baking sheets halfway through baking. Cool on baking sheets 1 minute. Remove to wire racks; cool completely.
4. For the Filling, beat butter, confectioners’ sugar, marshmallow cream & vanilla in medium bowl with electric mixer on medium speed until light and fluffy.
5. To assemble the Whoopie Pies, place 1heaping tablespoon filling on flat side of 1 cookie. Top with a second cookie, pressing gently to spread the filling. Roll or sprinkle side of whoopie pie with chopped nuts, sprinkles or color the cream with different food colorings. Repeat with remaining cookies, filling and decorations. Store prepared whoopie pies between layers of wax paper in airtight container in refrigerator for 3 to 5 days.