How to Make It
Preheat the oven to 350° and line 2 large baking sheets with parchment paper. In a medium bowl, whisk the flour with the cocoa powder, baking soda, baking powder and salt. In a large bowl, using a handheld electric mixer, beat the butter with the brown sugar at medium speed until fluffy, about 2 minutes. Beat in the egg and vanilla. At low speed, beat in the dry ingredients and buttermilk in 3 alternating additions; scrape down the side and bottom of the bowl as necessary.
Using a 3-tablespoon ice cream scoop, scoop 12 level mounds onto each baking sheet, about 1 1/2 inches apart. Bake the cakes in the lower and upper thirds of the oven for about 16 minutes, until risen and firm; shift the pans from top to bottom and front to back halfway through baking. Transfer the sheets to racks and let the cakes cool completely.
In a large bowl, beat the butter with the confectioners' sugar and vanilla seeds until fluffy. Beat in the Marshmallow Fluff. Scrape the filling into a pastry bag fitted with a large plain tip.
Pipe the filling onto the flat sides of 12 of the cakes and close the whoopie pies, pressing to evenly distribute the filling to the edges. Roll the edges in orange nonpareils and serve.
For ultra-dark, super chocolaty whoopie pies, substitute 2 tablespoons of black cocoa powder (available from kingarthurflour.com) for 2 tablespoons of the regular unsweetened cocoa powder called for here.