First prepare the chocolate pastry cream to have time to cool. Place milk in a medium saucepan and bring to a boil.
Meanwhile whisk the eggs with sugar and salt until well combined. Add cocoa powder and cornstarch and mix to combine.
Gradually add milk into the egg mixture and return to heat. Cook stirring constantly until it boils and thickens.
Remove from heat and add chocolate and stir until completely melted.
Pour the cream into a clean bowl, cover with plastic wrap, pressing it directly onto the surface of the pastry cream to prevent forming a skin on the surface. Let cool to room temperature.
Prepare chocolate choux pastry layers. In a saucepan bring the milk, water, sugar, salt and butter to a boil.
Remove from heat and add flour and cocoa powder all at once and incorporate mixing energetically with a wooden spoon until homogenous.
Return the saucepan over low heat and while stirring cook for 1 minute or two to pull out the moisture from the batter and until it pulls away from the sides of the pan. You will see some of the dough sticks to the bottom of the pan.
Transfer batter to a large bowl and allow to cool slightly.
Meanwhile preheat the oven to 375 F (190C). Grease two 8 inch (20cm) pans and line with parchment paper.
Add the eggs one at a time, carefully incorporating each into the batter, using a wooden spoon or even a stand mixer. It will result in a smooth, homogeneous batter which still holds its shape.
Divide choux pastry evenly and spread on the bottom of the pans.
Bake for 25-30 minutes until nicely puffed.
Prepare the chocolate filling. Place butter into a large bowl and mix until creamy. Gradually add spoons of pastry cream and mix to incorporate. Add another spoon only when the other spon is fully incorporated. Repeat until all the pastry cream is fully incorporated. In the end add vanilla extract and mix to combine.
Assembly. Add a bit of chocolate cream on a serving platter. Add the first layer of choux pastry. Place a ring and line with an acetate sheet for easier removal. Add chocolate cream on top of the pastry layer. Fill the gaps from the second layer of pastry and place on top of the chocolate cream. Cover and refrigerate for 4-6 hours or better overnight.
Before serving, dust with powdered sugar. Serve as is or with a berry coulis or chocolate sauce. Enjoy!