Preheat the oven to 425 F/218 C.
Begin by mixing the flour, sugar, and salt together in a bowl.
In a large pot add the water and the butter. Heat it on the stovetop with medium heat stirring occasionally and bring it to a boil.
Remove the pot from the heat and then add in the flour mixture all at once and stir quickly with a wooden spoon until a dough forms. Place it back on the heat and keep stirring cooking it for 30 seconds. Then remove from the heat.
Add the dough to a large bowl and stir it with the spoon for a few minutes. This will release the steam in the dough and cool it down. If it is too hot it will cook the eggs.
Now stir in each egg one at a time, until well blended. It will seem like the egg won't want to blend it, but just keep stirring and it eventually will, then repeat the process with the rest of the eggs.The choux pastry dough is ready to go.
Add the pastry dough to a piping bag with a large round tip or French star tip. Or just use a large ziplock bag with the corner snipped off.
Prepare a sheet pan with parchment paper or a silicone mat and then pipe on 1 inch wide by 4 to 5 inch long strips of choux pastry dough. Make sure the strips are about 2 inches apart.
Create an egg wash by mixing one egg and one teaspoon of water. Then baste the tops of the unbaked eclair shells. You can also use the basting brush to shape the eclairs and smooth out any piping mishaps.
Bake the eclairs in the oven, middle rack position, for 15 minutes, then turn the heat down to 350 F/176 C. And bake for an additional 20 to 25 minutes. DO NOT open the oven until 35 minutes total time has gone by otherwise they may deflate. Once they are nice and golden brown on the top and sound hollow they are ready to go. Allow them to cool while you make the pastry cream and ganache.
Now make the pastry cream by adding the pudding mix to a bowl with the milk. Stir and set aside for 5 minutes. In another bowl whip up the heavy cream until stiff peaks form using a mixer. Then fold in the pudding creating an easy pastry cream. You may also fold in an egg custard instead of pudding if you like.
Next, make the chocolate ganache. In a small bowl or mug heat up the heavy cream until it gets really hot, about 45 seconds in the microwave. Then pour it over a different bowl filled with the chocolate. Don't stir, just allow it to sit for 5 to 10 minutes and melt the chocolate. Then come in a stir it until smooth. Add more cream or more chocolate for the desired consistency.
Add the pastry cream to a pastry bag with an injector tip. Poke holes in the bottoms of the eclairs with a wooden skewer. Then stick the tip in and pipe in the pastry cream, until the eclair is full and feels heavy. Alternatively, you can just cut the eclairs in half lengthwise and fill them like a long cream puff.
Now just dip the filled eclairs in the chocolate ganache. And allow the excess to drain off into the bowl. Place them back on the sheet pan or on a wire rack to allow the ganache to set. Enjoy! Keep chilled in an airtight container in the fridge until ready to serve.