The whoopie pie — two soft, cake-like chocolate rounds sandwiching a thick layer of fluffy white marshmallow filling — is claimed by both Pennsylvania (where the Amish communities make them) and Maine (where they are the state treat). The name reportedly comes from the exclamation of delight when children found them in their lunch pails: "Whoopie!" The whoopie pie is not a pie, not a cookie, and not a cake — it occupies its own category of American baked goods: a handheld, portable, cake-sandwich format that predates the macaron trend by a century.
Two soft, round, dome-shaped chocolate cake discs (approximately 8cm diameter, 2cm thick) sandwiching a thick layer of marshmallow cream filling (a cooked or whipped mixture of marshmallow fluff, butter, powdered sugar, and vanilla). The cake should be moist, soft, and deeply chocolatey — more cake than cookie in texture. The filling should be fluffy, sweet, and white — the contrast between the dark cake and the white cream is the visual signature.
1) The cake rounds are baked, not fried — scooped onto a sheet pan and baked at 175°C for 10-12 minutes. They should be soft to the touch when done. 2) The filling must be fluffy — beat the butter and marshmallow fluff together until light. A dense filling makes the sandwich leaden. 3) The cake must cool completely before filling — warm cake melts the filling.
Pumpkin whoopie pies (pumpkin-spice cake rounds with cream cheese filling) are the autumn variation that has overtaken the original in some regions. Red velvet whoopie pies. Peanut butter filling instead of marshmallow. The format is endlessly adaptable.
William Woys Weaver — Pennsylvania Dutch Country Cooking