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