To make the cakes, preheat the oven to 350°F and line two baking sheets with parchment paper. Cream butter and brown sugar with an electric mixer until combined, then beat in chocolate and vanilla. Add eggs one at a time, beating well after each.
In a large bowl, whisk together flour, cocoa powder, baking soda, and salt. With the mixer set to low, add half the flour mixture to the butter mixture and beat. Mix in buttermilk and remaining flour mixture. Beat until combined.
Drop rounded tablespoons of batter onto the prepared baking sheets. Bake for 11 minutes. Cool on the baking sheet for 5 minutes, then move to a rack.
Meanwhile, make the filling: Beat butter with a mixer until smooth. Gradually add confectioners' sugar until combined. Add Marshmallow Fluff and vanilla and mix well. To assemble whoopie pies, sandwich a heaping tablespoon of filling between two cakes. Roll edges in sprinkles.