In a heavy-bottomed saucepan over medium-high heat, add Whole Milk (2 cup), Egg (4), Granulated Sugar (1/2 cup), Vanilla Extract (2 teaspoon), Corn Starch (4 tablespoon), and Salt (1 pinch). Whisk together. Cook for 3-8 minutes, or until mixture thickens.
Pass custard through a strainer into another dish. Cover in plastic wrap, making sure the plastic wrap touches the custard. Store in fridge.
In a saucepan, add Water (1 cup), Butter (1/2 cup) a bit more sugar, and Salt (1 pinch). Bring to a boil.
Add All-Purpose Flour (1 cup). Turn the heat down and stir the mixture until it comes together into a ball.
When the dough comes together, turn the heat to medium and cook for 2-3 minutes. Transfer to a bowl and allow to cool completely.
Add Large Egg (4) one at a time, mixing as you go. Transfer mixture to a piping bag.
On a baking pan lined with parchment paper, squeeze dough out into 3 to 4-inch lines. Brush each gently with water.
Bake at 425 degrees F (220 degrees C) for 15 minutes.
Reduce heat to 350 degrees F (175 degrees C) and bake for another 25 minutes.
Heat Heavy Cream (1/3 cup) in the microwave. Pour hot cream into a bowl with the Semi-Sweet Chocolate Chips (4 ounce) and allow them to melt together.
Transfer custard to a piping bag. Make a small hole in each eclair and fill it with the custard. Dip one side of each eclair into the chocolate.