Make the custard first: Heat milk until steaming. Whisk yolks, sugar, cornstarch, and salt. Temper eggs with hot milk, then return mixture to saucepan. Cook, whisking constantly, until thick and boiling. Boil 1 minute. Off heat, whisk in butter and vanilla. Strain, cover with plastic directly on surface, and refrigerate for at least 2 hours.

For the cookies: Preheat oven to 350°F (175°C). Line baking sheets with parchment. Whisk flour, cornstarch, baking soda, and salt in a bowl.

Cream softened butter and both sugars until light and fluffy. Add eggs one at a time, then vanilla. Mix in dry ingredients on low until just combined. Mix in sour cream.

Scoop 1.5-tbsp portions of dough onto sheets, 2 inches apart. Bake for 10-12 minutes until edges are set but tops are soft. Cool on sheet 5 mins, then transfer to wire rack to cool completely.

Assemble: Pair cooled cookies. Pipe or spread chilled custard onto the bottom of one cookie and sandwich with its pair.

Make ganache: Place chocolate chips in a bowl. Heat cream (and corn syrup) until simmering. Pour over chocolate, let sit 2-3 mins, then whisk until smooth.

Spoon warm ganache over the top of each assembled cookie, allowing it to drip down the sides. Let set at room temperature for 1 hour before serving.