First prepare the pastry cream to have it ready by the time the eclairs are baked.

Whisk the egg yolks with sugar until slightly pale.

Incorporate the cornstarch. Pour the milk into a saucepan and bring to a simmer. Pour about a third of the hot milk over the egg yolks mixture. Pour the whole mixture into the saucepan over the milk. Bring to a boil while stirring and thickens. Keep enough over heat to cook all the cornstarch. Add vanilla extract.

Pour the cream into a clean bowl and allow to cool for 10 minutes then incorporate the butter while stirring. Cover with plastic wrap, pressing it directly onto the surface of the pastry cream to prevent forming a skin on the surface. Refrigerate until ready to use.

Preheat oven to 350F (180C).

Prepare the choux pastry. Sift the flour. In a saucepan bring the milk, water, sugar, salt and butter to a boil.

Remove from heat and add flour 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. 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.

Fit a pastry bag with a large ½ inch (1 cm) round or star tip and pipe the dough into 4 or 5 inch (10 -12 cm) long strips on a parchment paper lined baking sheet.

Dust with powder sugar and bake for about 40 minutes. After the first 10 minutes, open the oven door slightly, about  1/8 inch (2-3 mm), to let the steam escape. Bake for the rest of 30 minutes with the oven door slightly ajar, until golden. (Use a wooden spoon to keep the oven door ajar)

Remove eclairs from the oven, prick each with a skewer to release steam and allow to cool on a wire rack.   Poke 3 holes in the base of each eclair using a 1/4 inch (5-6mm) plain or star tip.

Remove pastry cream from refrigerator and whisk until smooth. Transfer the pastry cream into a piping bag fitted with 1/4 inch (5-6 mm) plain or star tip.

Fill the cooled eclairs by piping cream into the holes.

Prepare the chocolate ganache. In a small saucepan, place cream over medium-low heat for a few minutes to get hot. No boiling or simmering needed. Remove from heat, pour over chopped chocolate and let rest for 1-2 minutes. Stir until dissolved.

Dip the tops of the eclairs into the chocolate and allow to set. You won’t need all the chocolate but needs to be more to be able to dip them well.

Eclairs are best eaten the day they are made but they keep quite well in the refrigerator overnight.