Preheat the oven to 350ºF. Line a large baking sheet with parchment or a silicone baking mat.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
In the bowl of a stand mixer fitted with the paddle attachment, beat together butter and sugar until creamy.
Add the egg and vanilla and beat until incorporated.
Alternate adding the flour mixture and buttermilk in about three parts each, starting with flour and ending with milk.
Pipe or spoon 2 inch rounds onto the prepared baking sheet. *(See note)
Bake 13-15 minutes.
Let cool for 5 minutes on the baking sheet, then transfer to a cooling rack to cool completely.
Repeat with any remaining batter.
Vanilla Pastry Cream
Whisk sugar, cornstarch, and salt together in a 2 quart saucepan.
In a glass measuring cup, whisk together milk and egg yolks.
Slowly whisk the milk mixture into the sugar mixture.
Cook over medium heat, stirring constantly, until boiling. Let boil 1 minute.
Remove from the heat and add the vanilla.
Let cool slightly before transferring to a bowl, pressing plastic wrap against the top of the cream, and refrigerating until completely cold, at least 2 hours.
Bring heavy cream to a boil in a small saucepan.
Remove from the heat, add the chocolate, and stir until completely melted.
Allow to cool 10 minutes before using.
Assembling the whoopie pies
Spoon about 2 tablespoons of pastry cream onto the bottom of one cookie.
Gently press the underside of another cookie against it to create a sandwich. Repeat with the remaining cookies and pastry cream.
Carefully spoon ganache over the top of each sandwich cookie, spreading it almost to the edges and maybe letting a little drip down over the sides.
Refrigerate until ready to serve. Whoopie pies will keep for up to 3 days in the refrigerator.
To pipe the cookie batter onto the baking sheet, place the batter in a plastic pastry bag (you could also use a plastic food storage bag) and snip the end off. Then pipe the batter into 2 inch rounds on a baking sheet lined with a silicone baking mat (you could also use parchment).
Tips for customizing
Make these gluten-free by using a gluten-free whoopie pie cake recipe (no need to change anything else since the Argo cornstarch keeps the cream gluten-free and the chocolate is naturally gluten-free!)
Add a couple fresh strawberry slices on top of the cream before sandwiching
Stir a bit of sweetened shredded coconut into the cream before using as a filling
Melt white chocolate and pipe it on top of the ganache to create beautiful designs or hand lettering