Preheat oven to 375ºF and place oven rack in the center of the oven. Line two baking sheets with parchment paper. In a large bowl, sift together the flour, cocoa powder, baking soda, baking powder and salt. In an electric mixer bowl fitted with the paddle attachment, beat the butter and sugar until light and fluffy. Add the egg, beating well. Beat in the vanilla. In a small measuring cup, mix the buttermilk and water. With the mixer on low speed, alternately add the flour mixture and buttermilk mixture, in three additions, beginning and ending with the flour. Drop heaping tablespoons (can also use a small cookie scoop) of the batter onto the prepared baking sheets, spacing about 2 inches apart. With moistened fingers, smooth the tops of the cookies. Bake for about 9 to 10 minutes or until the tops of the cookies, when lightly pressed, spring back. Remove from the oven and transfer to a wire rack to cool completely.
Beat the shortening and butter until soft and creamy. With the mixer on the lowest speed, gradually beat in the confectioner’s sugar. Increase the speed to high, and beat until light and fluffy, about 5 minutes. Then, with the mixer on low speed, beat in the maple flavor and slowly drizzle in the corn syrup. Continue to beat until the filling looks like soft mayonnaise. By hand, fold in marshmallow crème.
To assemble: Take one cookie and spread a heaping tablespoon of the filling on the flat side of the cookie. Top with another cookie.