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1
Preheat the oven to 350u00b0F. Line two baking sheets with parchment paper.
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2
Make the cakes: In the bowl of an electric mixer fitted with the paddle attachment, cream the butter, dark brown sugar, sugar, and molasses until light and fluffy, 4 to 5 minutes.
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3
Add the eggs gradually, and mix to combine. Scrape well after they're incorporated. Add the vanilla and mix to combine.
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4
In a medium bowl, whisk the flour, baking powder, and salt to combine. Add about 1/3 of the flour mixture to the butter mixture and mix to combine.
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5
Add half of the buttermilk to the butter mixture and mix to combine. Add another 1/3 of the dry ingredients, then the remaining buttermilk. Finally, mix in the remaining flour and mix to combine-don't overmix.
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6
Scoop heaping 1/3-cup-sized mounds of batter onto the prepared baking sheets, leaving at least 1 1/2 inches of space between them-they will spread a bit! Top each with a sprinkling of flaky salt.
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7
Bake until the cakes spring back lightly when touched in the center (or a toothpick inserted into the center comes out clean, 16 to 19 minutes.
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8
Let the cakes cool completely on the baking sheets. Make the filling while the cakes cool.
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9
In a medium pot, combine the water, dark brown sugar, corn syrup, and molasses to combine. Stir over medium heat until it comes to a boil. When it begins to boil, stop stirring and attach a candy thermometer to the pot (if any sugar crystals have washed up on the sides, brush them away using a pastry brush dipped in cool water). Continue to cook the syrup until it reaches 240u00b0F (final desired temperature).
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10
As soon as the sugar hits 230u00b0F, begin whipping the egg whites on medium-high speed. The goal is to have the egg whites at soft peaks when the sugar reaches the 240u00b0F.
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11
With the mixer running, add the sugar syrup to the egg whites in a slow, steady stream. Continue to whip on high speed until the mixture reaches stiff peaks and the bowl is no longer noticeably warm to the touch. The meringue should be smooth and glossy, not clumpy or dry.
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12
With the mixer still running, gradually add room temperature butter in 1/2-tablespoon chunks. Continue adding and mixing until all of the butter is incorporated and the mixture is light and smooth.
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13
Add the powdered sugar and mix to combine. Beat in the vanilla extract and salt (taste to see if you want a bit more salt!). Transfer the finished filling to a disposable pastry bag and cut 1/4-inch opening from the tip.
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14
Peel the cooled cakes away from the parchment paper and turn half of them upside down.
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15
Pipe filling into a spiral (no need to be perfect-just helps get it into an even layer) on the overturned cakes. Top with another cake. I like to refrigerate the finished whoopie pies for at least half an hour before serving, but you don't have to! It just helps the filling set a bit so it doesn't ooze out when you bite into it!
