These stout whoopie pies are cake-like, but sturdy enough to keep their shape. The confectioners’ sugar based buttercream filling is flavored with stout. These are simple to make. Just open up a bottle and get baking. The pies are best if served the same day they are baked, but they can be held overnight in which case I would assemble them and individually wrap in plastic wrap.
Preheat the oven to 350 degrees F. Position racks in upper and lower third of oven. Line two baking sheets with parchment paper and set aside.
In a large bowl, whisk together the flour, cocoa, baking soda, and salt until combined.
Beat the butter with an electric mixer in a large bowl until soft and creamy. Add both sugars and continue beating until light and fluffy, about 3 minutes, scraping down the bowl once or twice as needed. Beat in the vanilla and egg until combined. In a separate bowl, whisk together the stout and buttermilk and add this wet mixture alternately with the dry mix in a few batches. Beat batter just until combined. The mixture will be thick.
Scrape batter into pastry bag fitted with tip and pipe out rounds about 2 ½-inches in diameter. You should have 8 rounds per pan spaced out as evenly as possible to allow for spread.
Bake for about 10 to 15 minutes or until a toothpick inserted in the center has a few moist crumbs clinging. Cool pans on racks completely.
Beat butter until creamy with an electric mixer in a large bowl. Beat in confectioner’s sugar until it begins to combine. Add 1/3 cup stout and continue beating until very smooth. Only add more stout as needed for proper texture. The buttercream should be thick, but spreadable.
Spread buttercream on flat side of one whoopie pie and sandwich together with another. Repeat until done. Serve the same day or individually wrap in plastic wrap and store at room temperature for up to 24 hours.