Prepare the choux batter:
Preheat the oven to 400 F and line two sheet pans with a parchment paper.
In a medium non-stick pot combine water with salt and bring to a boil.
Add butter and bring to a boil one more time.
Add flour and whisk vigorously until no lumps remain and the batter thickens about 1 minute.
Remove pot from the heat and let the batter cool for 5 minutes.
Whisk in the first egg and mix with a wooden spoon until the egg is fully incorporated into the batter with no white lumps. Continue whisking in the remaining eggs, one at a time
Using a pastry bag fitted with a large plain tip (I use Wilton #1A), pipe fat lengths of dough (about ¾ – 1 inch thick and 4 inches long) onto the lined baking sheet, leaving 1 inch of space between them.
Bake for 15 minutes.
Without opening the oven door, reduce the temperature to 340 F and bake for another 15-20 minutes until eclairs are light golden brown. Remove from the oven and cool on the wire rack.
Prepare the pastry cream:
In a small bowl dissolve cornstarch in 3-4 tablespoons of milk.
To a small saucepan add the remaining milk and bring it to a simmer. Add 50 grams (1/4 cup) of sugar and stir until sugar dissolves.
When milk starts to boil, add dissolved cornstarch and whisk until no lumps remain. Remove from the heat and cool until just slightly warm or completely cold.
In a large bowl beat the butter with the remaining 50 grams (1/4 cup) of sugar and vanilla extract until the mixture turns almost white.
Combine the whipped butter with the milk and cornstarch mixture. Mix well and set aside.
In a large bowl and using the electric mixer, beat the heavy whipping cream until soft peaks form. Fold whipped cream into the butter and milk cream.
Using the tip of a small knife, cut 3 small Xs along the top of each pastry. Using a small tip (I used Wilton tip #32), pipe the pastry cream into the shells through each opening.
For the chocolate glaze:
Melt the chocolate chips and heavy whipping cream in a medium saucepan set over a pot with about 1 inch of boiling water. Whisk until the mixture is nice and smooth.
Alternatively, you can heat chocolate and cream in a microwave for about 30 seconds and whisk until smooth.
Remove from the heat and cool for a few minutes.
Dip the top of each eclair into the chocolate glaze and set aside to allow the glaze to harden
