Cookie Dough Whoopie Pies

Preheat the oven to 350 degrees F. In a large bowl, sift together flour, baking soda, cocoa powder, and espresso powder. Stir in sugar. Make a well in the center of the dry ingredients and stir in milk, oil, egg, and vanilla. Stir until flour is incorporated and no large lumps remain; mixture should have the consistency of thick cake batter. Line a baking sheet with a silicone mat. Drop batter by the tablespoonful approximately 2 inches apart. Bake 10 to 12 minutes, or until cookies are set. Let cool on baking sheet about 5 minutes, then transfer to a wire rack to cook completely. Repeat with remaining batter. To prepare filling, beat together butter and brown sugar in a large bowl with an electric mixer on meduim speed until light and fluffy, 2 to 3 minutes. Mix in the flour and salt on low speed until incorporated. Add marshmallow creme and vanilla and beat until fluffy. Stir in chocolate chips. To assemble whoopie pies, sandwich 1 heaping tablespoon of filling between two cookies. Press until filling spreads evenly to the edges. Repeat with remaining cookies. Refrigerated in an airtight container, whoppie pies will keep for up to 3 days.