Step 1: Make the Cookies

1. Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. In a large bowl, cream together butter and sugar until light and fluffy.
3. Beat in eggs and vanilla until smooth.
4. In a separate bowl, whisk together flour, baking powder, baking soda, and salt.
5. Alternate adding the dry ingredients and buttermilk to the wet ingredients, mixing until combined.
6. Drop tablespoon-sized scoops onto the baking sheet, leaving space between each.
7. Bake for 10–12 minutes until lightly golden. Let cool completely.

Step 2: Make the Pastry Cream Filling

8. In a saucepan, heat milk over medium heat until steaming (not boiling).
9. In a separate bowl, whisk together sugar, egg yolks, and cornstarch until smooth.
10. Slowly whisk the hot milk into the egg mixture, then return everything to the pan.
11. Cook over medium heat, whisking constantly, until thickened.
12. Remove from heat, stir in vanilla and butter, then cool completely.

Step 3: Make the Chocolate Ganache

13. Heat heavy cream in a small saucepan until hot (but not boiling).
14. Pour over chopped chocolate, let sit for a minute, then stir until smooth.

Step 4: Assemble the Cookies

15. Spread or pipe a teaspoon of pastry cream onto the flat side of a cookie.
16. Top with another cookie to make a sandwich.
17. Dip the tops into chocolate ganache and let set.