Heat the oven to 375°F. In a medium saucepan, combine the water, salt, and butter. Cook on medium-low heat, stirring occasionally. Once all butter is melted, dump in all the flour and vigorously mix with a wooden spoon.

Reduce the heat to low and continue cooking, stirring constantly, until the mixture becomes a sticky paste and there’s a visible film of starch on the bottom of the pan, for 2 to 3 minutes.

Transfer the mixture to a mixing bowl. Cool for 10 mins, until room temp. Add an egg to the paste and vigorously mix using a fork or Dutch whisk. The pastry will first look clumpy and separated, but the more you mix, the smoother it will get.

Once the first egg has been incorporated, add in the second egg and mix until combined.

Line a large baking sheet with parchment paper. Trace 2 x 8.5-inch squares on the parchment paper. Flip the parchment on the other side and set aside.

Add the pastry to a piping (or ziplock) bag. Cut the tip off and equally squeeze the pastry in both squares. Spread the pastry all over the square surface using a spatula.

Place the pan on the middle rack of the oven and bake for 18 to 23 minutes, or until lightly golden yellow and hard to the touch. Pull the pan out the oven, place the pastry squares on a cooling rack and set aside.

In a medium mixing bowl, stir together the yolks, sugar, cornstarch, and salt. In a medium saucepan, combine the milk and vanilla beans over medium heat and heat until hot and close to simmering.

While constantly whisking the egg mixture, slowly stream 2 cups of the hot vanilla milk. Pour this warm yolk mixture into the pot with the rest of the milk. Cook over medium-low heat for 5 to 8 minutes, whisking constantly, until the mixture starts bubbling and becomes visibly thickened, like pudding.

Remove the pot off the heat, then mix in the cubed butter until combined. Strain through a fine-mesh sieve to catch any clumps or overcooked egg.

Place a baked choux pastry square at the bottom of a 9-inch square pan. If needed, use scissors to trim the border of the choux squares so they can fit inside the baking dish. Pour all the (still warm) pastry cream over the choux square. Top with the other choux square, directly on the pastry cream. Set aside.

In a medium saucepan over medium heat, bring the cream to a simmer. Remove off the heat, add the chocolate and salt. Wait a few minutes, then whisk until silky smooth.

Gently pour the chocolate ganache all over the cake, using a ladle or spoon. Chill in the fridge overnight, for at least 12 hours before serving. Slice and serve!