Start by making the vanilla filling first so it has plenty of time to set and chill while you make the éclair shells.
In a small bowl or 4-cup measuring cup, beat the instant vanilla pudding mix and powdered sugar into 1 cup of milk until it begins to thicken.
Beat in the container of cool whip.
Cover the bowl with plastic wrap and put in the fridge.
Now for the choux paste...In a medium pot over medium-high heat on the stovetop, bring the water, butter, and salt to a boil.
Once boiling, add the flour all at once and quickly stir until the mixture comes together in a ball.
Remove from the heat and let sit for 10 minutes to cool slightly.
Preheat oven to 400 degrees and line a baking sheet with parchment paper.
Crack the eggs into a small bowl or cup.
Add the eggs to the cooked choux paste mixture in the pot one at a time, stirring quickly with addition to incorporate the eggs completely. (At first, the eggs will seem resistant to being mixed in but just keep going. They'll mix in.)
Scoop the choux paste into a 16-inch disposable frosting bag.
Cut off enough of the end of the bag to be able to pipe a strip of choux paste that is as big as a regular-sized hot dog.
Pipe 3 ½ long strips of choux paste onto the prepared baking sheet, leaving a little less than an inch between each one. You should get all 20 eclairs on each pan.
Bake in the preheated oven (400 degrees) for 18-20 minutes. DO NOT open the oven door at any point during baking to check on them!!! This will release the steam in the pastries, and they'll go flat.
The eclair shells should be golden brown.
Cool on a wire rack.
While the eclair shells are baking, make the ganache.
Melt the chocolate in the microwave or in a melter pot with the heavy cream.
Stir until completely smooth.
Allow the ganache to cool until it becomes thick and pipeable while you fill the eclairs.
To fill the eclairs use something long and thin like a chopstick, to carefully poke a small hole into one end of each eclair. Wiggle the chopstick around a little bit to make room for the filling.
Fill a disposable frosting bag fitted with a #10 tip with vanilla filling.
Gently poke the tip in each hole and fill each eclair.
Fill a disposable frosting bag fitted with your preferred tip with ganache. Pipe a strip of ganache over the top of each eclair.
Store the eclairs in a loosely covered container in the fridge.