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.
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.
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.
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