Place the milk, about half the sugar, and the salt into a saucepan and heat over medium heat until simmering but not boiling.

Meanwhile, in a heat-proof mixing bowl whisk together the remaining sugar and cornstarch and then stir or whisk in the egg yolks until smooth. It will be fairly thick.

Once the milk is simmering, remove from the heat and then very gradually whisk in about ½ cup of it to the egg yolk mixture. Whisk vigorously to temper the eggs so they don’t scramble. Pour the egg mixture back into a saucepan with the remaining milk.

Return the saucepan to medium-low heat and continue whisking the mixture until it thickens, about 5 to 9 minutes. Don’t stop whisking or it can get lumpy and curdle.

Remove from the heat and whisk in the vanilla.

Transfer the mixture to a large shallow container. The larger surface area will help it cool faster. Cover with plastic wrap so the plastic is pressed against the surface of the custard (to prevent a skin from forming). Cool to room temperature. The custard needs to be at room temperature (not cold) for the later steps.

Preheat the oven to 400°F (200°C) with the rack in the center. Grease the bottom and sides of two 9-inch springform pans with butter and line the bottoms with parchment paper circles.

Add the water, butter, and salt to a medium saucepan over medium heat. Once the butter has melted, remove the pan from the heat and stir in the flour all at once.

Reduce the heat to medium-low and return the pan to the heat. Stirring constantly, continue to cook until the dough forms a ball and leaves a white residue on the bottom.

Transfer the dough to the bowl of an electric stand mixer fitted with the paddle attachment. Let the mixture cool for about 10 minutes.

With the mixer running, add one egg at a time until fully incorporated. The resulting batter should be smooth and glossy.

Divide the dough between the prepared pans. Spread the tops of the dough unevenly using the back of a spoon to create some peaks and divots.

Bake for about 24 to 28 minutes until it’s puffed and evenly golden brown across the top. Remove from the oven and cool for 10 minutes in the pans. Then remove from the pans and cool completely on a wire rack.

In the bowl of a stand mixer fitted with the whisk attachment, beat together the soft room temperature butter and sugar until pale and fluffy.

Beat the crème pâtissière in its own bowl just to loosen it up and make sure it’s nice and smooth without any lumps. Gradually add the crème pâtissière to the butter mixture a couple of spoonfuls at a time.

Continue beating until the mixture is light and fluffy, about 5 minutes.

Place one of the two choux pastry layers inside the ring. Spread the vanilla custard over the top evenly, and then top with the remaining layer of choux pastry.

Refrigerate for at least 1 hour but up to 3 hours to allow the filling to set. Remove the ring, peel off the parchment paper, and dust the top with powdered sugar before serving.