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1
Adjust the racks to the upper and lower thirds of the oven and preheat the oven to 350F.
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2
Line two baking sheets with parchment paper.
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3
To make the cakes: Combine the cocoa and milk in a medium saucepan, set over low heat, and whisk to create a smooth paste.
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4
Add the butter, sugar, and corn syrup and continue to whisk until the mixture is smooth and glossy and the sugar has dissolved, about 3 minutes.
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5
Remove the saucepan from the heat and let cool.
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6
Whisk the eggs into the cooled cocoa syrup.
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7
Place the flour, baking powder, baking soda, and salt in the bowl of a standing mixer fitted with the paddle attachment or another large bowl.
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8
With the mixer on medium speed, or beating with a hand mixer, slowly pour the cocoa syrup into the flour mixture, mixing until combined.
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9
Transfer the batter to a pastry bag fitted with a large plain tip and pipe out 2-inch rounds spaced 3 inches apart onto the baking sheets, 12 per pan.
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10
Place both pans in the oven and bake until the cakes puff and are set, about 10 minutes, rotating the pans top to bottom and back to front after 5 minutes.
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11
Remove from the oven and slide the parchment sheets with the cookies onto a counter.
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12
Cool for 2 minutes, then transfer the cakes to a cooling rack with a metal spatula and cool completely.
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13
To make the filling: Beat the butter in the bowl of the standing mixer, fitted with the paddle attachment, or in a large bowl with the handheld mixer, until fluffy about 1 minute.
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14
Add the sugar and beat until the mixture is light and fluffy, about 2 minutes more.
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15
Add the Marshmallow Fluff, vanilla, and salt and beat to combine, about 30 seconds.
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16
To assemble the whoopie pies, turn half the cakes onto their backs and spread each with a generous portion of the filling.
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17
Place a second cake on the top of each to make a sandwich.
