In a sauce pan, heat up the milk to simmer or till you see bubbles around the edges. Turn of the heat and let it sit. Note: If using vanilla pod add it to milk and let it infuse for 10 minutes.

Meanwhile in a bowl, cream together egg yolk and sugar till it becomes pale yellow in color. Also whisk in corn starch thoroughly. 

Take 1/4 cup of hot milk and temper it to egg mixture whisking continuously.  Slowly keep adding and whisking  1/4 - 1/2 cup milk. Once all the milk is incorporated, pour the mixture back in same pan. Turn the heat on medium and cook the mixture by constantly whisking until it thickens. This takes around 2-3 minutes.

Remove from the heat and let it cool slightly.

Transfer it to a bowl. Cover it with plastic wrap, pressing the plastic against the surface to prevent any skin formation.

Let it chill in the fridge for couple of hours before using. It can also be made a day in advance.

Preheat the oven at 425 degrees. Line 2 sheet pan with parchment paper. Prepare a piping bag with any big piping tip ( star tip # 1M or round tip # 1A).

In a sauce pan, boil water and butter on medium heat. Once water starts boiling turn off the heat and stir in all purpose flour. Using a wooden spoon completely incorporate the flour. 

Turn on the heat, and cook the flour mixture for a minute or until raw flavor of flour is gone.

Transfer the mixture to stand mixer bowl with paddle attachment or any big bowl if using hand mixer.

Turn on the mixer and let it go for 1 minute. You will see steam escaping which will eventually cool down the dough. 

With mixer on, start adding eggs one at a time. Scrape the mixture in between. Mix until the mixture is smooth and all the eggs are incorporated completely.

Fill up the dough in the piping bag and pipe out 3-5 inches logs every inch apart on the prepared baking sheets.

Brush the top of those logs with some water and bake them for 10 minutes. Reduce the temperature at 350 and continue baking for 20 minutes.

Once they are baked and still hot, take a skewer or piping tip and make a hole on both the sides so steam can escape.

Allow it to cool completely before filling them with pastry cream.

In a sauce pan, bring heavy cream to simmer.

Pour hot heavy cream on chocolate chips and let it sit for 2 minutes. Whisk the mixture until the chocolate has melted.

Fill up pastry cream to a piping bag fitted with a round tip ( tip # 12).

Fill the eclairs from the holes made at the end of each side with pastry cream.

Dip each eclair into ganache and let it sit for a while.

Other way of assembling is to cut the eclair into half and spoon some pastry cream in between (something like a sandwich).