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1
Preheat oven to 350 degrees.
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2
Butter two 4-inch muffin top pans.
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3
Line each muffin cup in the pan with a parchment paper round, butter parchment, and set aside.
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4
Cream together butter and sugar in the bowl of an electric mixer fitted with the paddle attachment until combined, 2 to 3 minutes.
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5
Add salt, eggs, and vanilla, and beat just until combined.
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6
In a small bowl, mix together baking soda and buttermilk.
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7
Add flour to mixer, alternating with buttermilk mixture, beginning and ending with flour.
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8
In a medium bowl, whisk together cocoa powder and boiling water until smooth.
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9
Add to batter and beat until combined.
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10
Fill each muffin cup with a heaping 1/4 cup batter.
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11
Transfer muffin pans to oven and bake 9 minutes, rotate pans, and continue baking 9 minutes more.
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12
Let cool slightly before transferring muffin tops to a wire rack.
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13
Let cool completely; remove parchment paper.
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14
Repeat process with remaining batter.
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15
FILLING:
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16
In the bowl of an electric mixer fitted with the paddle attachment, cream together butter and sugar until pale and fluffy, about 3 minutes.
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17
Add Fluff and vanilla and continue mixing until well combined.
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18
Place a heaping 1/4 cup filling on the bottom side of half of the muffin tops.
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19
Top with remaining muffin tops, bottom side down, to form whoopie pies.