For the choux pastry:

1. Preheat the oven to 480°F (250°C) et line a large baking sheet with parchment paper. Place a large mixing bowl in the refrigerator.
2. In a medium saucepan, heat water, milk, salt, sugar and butter on low heat. Remove the saucepan from the heat as soon as the liquid starts bubbling. Add the sifted flour and mix vigorously with a wooden spatula until no lumps remain.
3. Put the saucepan back on the heat and stir the dough until it detaches from the sides of the pan and makes a compact and smooth ball of dough. Transfer to the cold large mixing bowl and let cool for 5 minutes in the bowl placed at room temperature. Incorporate the eggs, one at a time, using a wooden spatula or a stand mixer equipped with a paddle attachment, stirring well after each addition.
4. Transfer the choux pastry in a large piping bag equipped with an Ateco #869 piping tip and pipe 4-inch (11 cm), leaving enough space between each of them. Bake in the 480°F (250°C) preheated oven, then drop the oven temperature to 350°F (180°C) right away, and bake for 25 minutes (do not open the oven in the meantime!), or a little bit longer if needed, until the pastry choux become golden in color. Open the oven door and let the choux pastry 5 more minutes in the oven, then transfer to a cooling rack and let cool completely.

For the chocolate pastry cream:

1. In a medium saucepan, heat the milk and the heavy whipping cream (turn off the heat before it starts bubbling).
2. Meanwhile, whisk the yolks with the sugar, sifted flour and cornstarch. Pour in the heated milk and mix well.
3. Put the mixture back in the saucepan and heat until it thickens, stirring constantly.
4. Melt the chocolate in the microwave, then add it to the cream off the heat, stir well, cover with cling film making contact with the cream, and place in the refrigerator for about 30 minutes before using it.

To assemble the eclairs:

1. Using a sharp knife, make three cross at the back side of the eclairs.
2. Fill a piping bag with a pastry tip size 5 with the cooled chocolate pastry cream and garnish the first hole and the last one with the ganache. Stop when the ganache is coming out from the middle hole, then smooth it out to remove the excess of ganache.

For the ganache:

1. Melt chocolate in the microwave.
2. Heat the heavy whipping cream in a small saucepan (do not overheat or it will burn the chocolate later), then pour over the melted chocolate in 3 additions, stirring well between each of them. Add in sugar and stir well.
3. Dip the eclairs into the chocolate ganache just enough to cover the top part, then place on a tray and let cool in the refrigerator for at least 2 hours before serving.