For the choux pastry, put water, butter, sugar, and salt in a saucepan. Bring to a boil and add the flour all at once, mix vigorously with a spatula until all the flour is gone and cook for another 2 minutes so that the dough dries out a bit. Pour into a bowl and mix in the eggs one by one until you have a nice smooth dough.
Take the measurements of your square baking pan and cut out a piece of cardboard or draw the size on baking paper. Do this twice. Place these cardboard patterns under your baking paper as a reference. Then pipe 2 square pieces of choux pastry according to the pattern, smoothing it out. Remove the cardboard patterns from under the baking paper.
Bake the choux pastry for 25 minutes in a preheated oven at 180°C. Turn it over and bake for another 2 minutes. Your dough will have very funky shapes but let them cool with a rack on top so that they flatten out again, trim the edges to match the cardboard patterns. Place one choux pastry in the form.
For the vanilla filling, mix the sugar, cornstarch, eggs, and vanilla seeds in a bowl and whisk until a bit frothy so everything is nicely combined. Put the milk, cream, and scraped vanilla pod in a saucepan and bring to a boil. Once it starts to boil, pour half into the egg mixture. Mix everything well and then put everything back together in the saucepan and set on heat. Keep whisking all around until it starts to 'pop'. Remove from heat and whisk the butter in well. Pour into the form and spread well against the sides. Press the other cut square choux pastry on top.
For the ganache: put the chocolate in a bowl. Heat the cream and pour it over the chocolate. Let it sit for 2 minutes and then mix until you have a nice smooth chocolate sauce. Pour the chocolate sauce over the choux pastry, smooth it out evenly, and let it cool overnight in the refrigerator.
With a warm knife, which you keep dipping in boiling water and drying off, cut the sides loose and then into 9 equal pieces. Serve with a touch of salt flakes, delicious!
