Make the Pastry Cream

In a heatproof bowl, whisk egg yolks, sugar, and cornstarch until thick and pale.

Heat milk in a saucepan over medium heat until steaming (do not boil).

Slowly pour the hot milk into the egg mixture while whisking constantly.

Return the mixture to the saucepan and cook over medium heat, whisking until thickened.

Remove from heat and stir in butter and vanilla.

Transfer to a bowl, cover with plastic wrap touching the surface, and refrigerate for at least 1 hour.

Make the Cookies

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

Cream butter and sugar together until light and fluffy.

Beat in the egg and vanilla extract.

Whisk together the flour, baking powder, and salt; add the dry ingredients to the wet mixture.

Add 2–3 tbsp milk to form a soft, scoopable dough.

Scoop tablespoon portions onto the baking sheet.

Bake for 10–12 minutes, until lightly golden around the edges.

Cool completely before assembling.

Make the Ganache

Heat heavy cream until steaming.

Pour over chocolate chips and let sit 1–2 minutes.

Stir until smooth and glossy.

Let thicken slightly before using.

Assemble the Cookies

Spoon or pipe chilled pastry cream onto the center of each cookie.

Drizzle or dip the tops in chocolate ganache.

Chill 10–15 minutes to set before serving.