1. Preheat oven to 350°F (176°C). Spray a regular sized cupcake pan with non stick cooking spray.
2. Cream butter and sugar together until light and fluffy, about 3-4 minutes.
3. Mix in egg and vanilla extract.
4. Combine the flour, baking soda, cornstarch and salt in a medium sized bowl, then add to the butter mixture. Mix until well combined and a dough forms. Dough will be thick and a bit sticky.
5. Makes balls of about 2 tbsp of dough. Press cookie dough into the bottom and about 1/3-1/2 way up the sides of each cupcake cup, forming a cup shape.
6. Bake for 10-12 minutes, or until edges are just golden. If they look a little undercooked in the center, that’s ok. They’ll continue to bake as they cool and firm up.
7. Remove from oven and allow to cool mostly in the pans, then remove to a cooling rack. If the centers aren’t cupped enough, use the bottom of a tablespoon to press the center down a bit while still warm.
8. While the cookies cool, make the pastry cream. Put the egg yolks in a medium sized bowl and gently beat them together. Set aside.
9. Add the sugar, cornstarch and milk to a large saucepan and mix until smooth.
10. Cook, stirring continuously, over medium-high heat until mixture begins to thicken and bubble.
11. Reduce heat to medium and simmer for 2 minutes. Remove from heat.
12. Add a little bit of milk mixture to the egg yolks and whisk together, then add egg mixture to milk mixture. This process ensures that you don’t cook the egg yolks.
13. Place the pan back on the heat and bring to a light boil. Allow to boil for 2 minutes, stirring continuously.
14. Remove from heat and add butter and vanilla extract. Stir until smooth, then top with clear wrap that touches the top of the cream (it helps prevent a film from forming on top). Refrigerate until cool.
15. To make the chocolate ganache, put the chocolate chips in a heat proof bowl.
16. Microwave the heavy whipping cream and corn syrup until it just begins to boil, then pour it over the chocolate chips. Allow it to sit for 2-3 minutes, then whisk until smooth.
17. Set ganache in the fridge until firm.
18. When you’re ready to build the cups, fill each cookie with about a tablespoon of pastry cream, then top with the chocolate ganache. If the ganache is a touch firm when you remove it from the fridge, let it sit on the counter a bit to warm up, or microwave it in 5 second increments until it’s a good consistency for piping.
19. Refrigerate the cookie cups in an air tight container until ready to serve.