For the choux pastry, put water, butter, sugar, and salt in a saucepan. Bring to a boil and add all the flour 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 get a nice smooth dough.
Take the measurements of your square baking pan and cut out a cardboard template or draw the size on baking paper. Do this twice. Place these cardboard templates under your baking paper as a reference. Then pipe 2 square pieces of choux pastry according to the template, smoothing it out. Remove the cardboard templates from under the baking paper.
Bake the choux pastry for 25 minutes in a preheated oven at 180°C. Turn them over and bake for another 2 minutes. Your dough will have very funky shapes but let them cool with a rack on top so they flatten out again, trim the edges to match the cardboard templates. Place one choux pastry in the mold.
For the vanilla filling, mix the sugar, cornstarch, eggs, and vanilla seeds in a bowl and whisk until 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 boiling, pour half into the egg mixture. Mix everything well and then put it all back in the saucepan and heat. Keep whisking all around until it starts to 'pop'. Remove from heat and whisk the butter in well. Pour into the mold and smooth it out to the edges. 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 in the refrigerator overnight.
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!
