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1
Make the pastry cream In a medium saucepan, whisk the milk with the sugar, egg yolks, cornstarch and salt; bring to a boil over medium heat, whisking frequently.
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2
Simmer until thickened, about 1 minute.
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3
Remove from the heat and whisk in the butter and vanilla extract until smooth.
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4
Transfer the pastry cream to a medium bowl.
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5
Press plastic wrap onto the surface of the pastry cream and refrigerate until chilled, about 2 hours.
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6
Meanwhile, make the cookies Preheat the oven to 350 and line 2 baking sheets with parchment paper.
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7
In a medium bowl, sift the flour with the baking soda and salt.
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8
Meanwhile, make the cookies In another medium bowl, beat the butter and sugar with a handheld electric mixer until fluffy, 3 minutes.
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9
Beat in the egg and vanilla.
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10
Beat in half the flour mixture, then all the buttermilk, followed by the remaining flour mixture, beating until smooth.
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11
Meanwhile, make the cookies Arrange twelve 1/4-cup scoops of batter 2 inches apart on the prepared baking sheets; using an offset spatula, spread the batter into 3/4-inch-thick rounds.
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12
Refrigerate for 15 minutes.
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13
Meanwhile, make the cookies Bake the cookies for 12 to 15 minutes, rotating the baking sheets halfway through, until a toothpick inserted in the center comes out dry.
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14
Let the cookies cool on the baking sheets for 5 minutes, then transfer them to a rack to cool completely.
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15
Make the chocolate glaze In a small bowl set over a saucepan of barely simmering water, whisk the chocolate with the butter and corn syrup until smooth.
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16
Let cool slightly.
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17
Make the chocolate glaze To assemble, arrange 6 cookies flat side up on a work surface.
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18
Beat the cold pastry cream with a wooden spoon and spread 3 tablespoons of cream onto each cookie.
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19
Top the cream with another cookie.
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20
Using an offset spatula, spread the chocolate glaze on top of each whoopie pie, letting it drip down the sides.
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21
Before serving, let the pies stand until the glaze is firm, about 1 hour.
