In a medium-sized pan, add the water, unsalted butter, 1 tablespoon of sugar, and salt, and cook on low heat until the butter is melted.
Then, turn the heat to high until a rolling boil is reached.
As soon as the water starts to rise, turn off the heat, add the flour all at once, and start to mix with a stiff spatula. The finished texture will eventually look like "mashed potatoes."
Once the "mashed potato" texture is reached, turn the heat back to medium, and continue mixing for 1 minute. Then, cut the heat.
Crack 2 eggs in a bowl, and mix with a fork.
Slowly add the eggs to the "mashed potato" mixture in 2 steps to avoid mixing for too long. Add 1/2 of the eggs at a time, and mix until the eggs disappear into the dough. Repeat the same steps until all of the eggs are incorporated. The dough is ready when it is smooth and shiny.
Preheat oven to 350 F.
Pipe mini eclairs on a nonstick baking tray or silicone mat using a size PF 16 pipping tip or a round pipping tip size 12 for mini eclairs, or size 15 for full sized eclairs.
Bake for 35 minutes, making sure not to open the oven during the bake time.
To prepare the chocolate cream, whisk together 4 tablespoons of sugar with the cornstarch.
Add 1 egg to the sugar and cornstarch, and whisk until smooth.
Bring the milk to a boil, and temper the egg mixture by adding 1/2 of the hot milk to the egg mixture while continuously stirring for 10 seconds. Add the tempered egg mixture back into the pot with the other half of the milk, and stir until boiling point is reached.
Add 1/2 cup of dark chocolate to the hot custard, and mix until the chocolate is fully incorporated.
Pour the hot chocolate custard on a large plate, and cover with plastic wrap. Chill in the refrigerator until cold.
Make 3 small holes on the bottom of the cooled down eclairs.
Whisk the chocolate cream until smooth before using.
Pipe the chocolate cream into the eclairs.
Heat the heavy whipping cream to a boil, and pour over the remaining dark chocolate chunks. Mix until smooth.
Dip the filled eclairs into chocolate ganache to glaze.