Cakes: 2 cups (250 g) all-purpose flour 1 / 2 cup (50 g) unsweetened cocoa powder 1 tsp baking powder 1 tsp baking soda 1 / 2 tsp fine salt 1 cup (200 g) granulated sugar 1 / 2 cup (110 g) packed light brown sugar 1 / 2 cup (120 ml) vegetable oil 2 large eggs, at room temperature 2 tsp pure vanilla extract 1 cup (240 ml) buttermilk (or 1 cup milk + 1 tbsp white vinegar, rested 5 minutes) Filling: 1 / 2 cup (115 g) unsalted butter, softened 1 3 / 4 cups (210 g) powdered sugar 1 (7 oz / 200 g) jar marshmallow creme (Fluff) 1 tsp pure vanilla extract 1–2 tbsp (15–30 ml) milk or heavy cream Pinch of fine salt
Heat oven to 350°F. Line 2 large baking sheets with parchment paper. This recipe yields about 12 whoopie pies (24 cakes). Whisk the dry ingredients: In a medium bowl, whisk flour, cocoa, baking powder, baking soda, and salt until evenly combined. Mix the wet ingredients: In a large bowl, whisk granulated sugar, brown sugar, oil, eggs, and vanilla until smooth. Whisk in the buttermilk until fully combined. Make the batter: Add the dry ingredients to the wet and whisk or fold just until no dry streaks remain; the batter will be thick and scoopable. Let it sit 5 minutes to slightly thicken. Portion and bake: Using a 1.5–2 tablespoon scoop, drop 24 even mounds onto the prepared sheets, spacing well. Smooth tops with a damp fingertip if needed. Bake 10–12 minutes, rotating pans halfway, until the cakes spring back when lightly touched and a toothpick comes out with a few moist crumbs. Cool: Let cakes cool on the sheets for 5 minutes, then transfer to a rack to cool completely. Make the filling: In a mixing bowl, beat the softened butter with a hand mixer until creamy, 1 minute. Beat in powdered sugar and salt until fluffy. Add marshmallow creme and vanilla; beat until smooth and airy, 1–2 minutes. Beat in just enough milk/cream to reach a thick, spreadable consistency. Assemble: Pair cakes by similar size. Spread or pipe 2–3 tablespoons filling on the flat side of half the cakes; cap with the remaining cakes, pressing gently to sandwich. Serve and store: Serve immediately or chill 20 minutes to set. Store covered at room temperature up to 1 day or refrigerate up to 4 days. For soft texture, let chilled pies sit 15 minutes before serving. Pies can be wrapped and frozen up to 2 months.
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