Preheat the oven to 200 C / 400 F and line a sheet pan (33 x 46 cm / 13 x 18 inch) with parchment paper.

Roll the puff pastry to about 3 mm thick if it's not pre-rolled. Using an 8 cm round cutter, cut 12 rounds total.

Use a 5 cm round cutter to cut the center out of 6 of the rounds, turning them into rings. Set the small centers aside.

Brush the 6 solid rounds lightly with beaten egg, avoiding the edges. Set a pastry ring on top of each round and press gently so the edges line up, then brush the tops with egg wash.

Transfer the stacked pastry to the fridge and chill for 15 minutes so the butter firms back up before it hits the oven.

Bake for 18 to 20 minutes, rotating the pan once, until the pastry is puffed and deep golden brown all the way through. Cool completely on a wire rack.

While the pastry bakes, heat the milk and vanilla in a 2-quart / 1.9 L saucepan over medium heat until steaming, not boiling.

Whisk the egg yolks, sugar, cornstarch, and salt together in a bowl until smooth and pale.

Slowly pour about a third of the hot milk into the yolk mixture, whisking constantly, then pour everything back into the saucepan.

Cook over medium-low heat, whisking constantly, for 3 to 4 minutes until the custard thickens enough to coat the back of a spoon. Remove from heat and whisk in the butter.

Strain the pastry cream through a fine-mesh sieve into a bowl. Press plastic wrap directly onto the surface and chill for at least 45 minutes, until fully cold.

Once the shells and pastry cream are both cold, spoon or pipe the pastry cream into the center of each shell, filling it just above the rim.

Sprinkle about 1/2 tablespoon of granulated sugar evenly over the top of each filled pastry.

Caramelize the sugar with a kitchen torch, moving in small circles until it turns deep amber, or run under a hot broiler for 1 to 2 minutes, watching closely since it can burn in seconds.

Let the caramel set for 1 to 2 minutes before serving.