Whoopie Pies

In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.

In a large bowl, whisk the brown sugar and vegetable oil together until smooth. Vigorously whisk in the egg and vanilla extract.

Alternately add the dry ingredients and the buttermilk to the wet mixture, beginning and ending with the dry ingredients. Mix until the batter is smooth and thick.

Use a cookie scoop to drop 2-tablespoon mounds of batter onto the baking sheets, spacing them 3 inches apart. Bake for 10-12 minutes until the cakes spring back when lightly touched. Let cool completely on a wire rack.

While the cakes cool, make the filling: beat the softened butter and powdered sugar until light and fluffy. Beat in the marshmallow fluff and vanilla until smooth. Pipe or spread a generous dollop of filling onto the flat side of half the cakes, then top with the remaining halves to form sandwiches.