1. Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.

2. In a bowl, cream together the butter and sugar until light and fluffy. Add the egg and vanilla; mix well.

3. Whisk together the flour, baking powder, and salt. Gradually add to the wet mixture and mix until combined.

4. Scoop dough by tablespoonfuls onto the prepared sheet and flatten slightly.

5. Bake for 8–10 minutes, or until lightly golden around the edges. Cool completely.

6. To make the custard, heat the milk in a saucepan until steaming.

7. In a separate bowl, whisk egg yolks, sugar, cornstarch, and salt. Slowly whisk in the hot milk.

8. Return the mixture to the pan and cook over medium heat, stirring constantly, until thickened. Remove from heat and stir in vanilla. Cool completely.

9. For ganache, heat cream until simmering. Pour over chocolate chips and let sit for 2 minutes. Stir until smooth.

10. Assemble by spooning cooled custard onto one cookie, topping with another cookie, and drizzling ganache over the top. Chill to set.