In a medium saucepan, combine the water, butter, sugar, and salt over medium-high heat.

Cook until the butter is melted, then bring to a full rolling boil over high heat.

Add the flour all at once and stir with a sturdy wooden spoon until everything comes together in a clump that is like a thick paste that cleans the sides of the pan.

Remove from the heat and stir in the eggs, one at a time. The mixture may seem to break or curdle at first, but keep stirring and it will come back together by the time you add the last egg. Let the choux pastry cool for 15 minutes.

Fill a piping bag with a large open star or french star tip with the choux pastry and pipe 16-18 strips, each about 4 inches long and 3/4 to 1" thick, spacing them about 1 1/2 inches apart, onto a parchment lined baking sheet. Do your best to pipe them evenly so that they are an even thickness all the way from end to end, rather than thick on the ends and narrow in the middle.

Turn off the oven and crack open the oven door to let the cream puffs cool down slowly for 10 minutes before removing from the oven. This prevents them from deflating.

Combine the milk, cream, and half of the sugar in a medium sauce pan over medium-high heat, stirring occasionally until the liquid comes to a simmer.

As the liquid heats, whisk together the remaining sugar, salt, cornstarch, and egg yolks in a medium bowl until light and creamy.

Once the milk mixture is hot, slowly whisk about 1 cup of the liquid into the egg mixture to temper the yolks. Pour the tempered egg mixture back into the saucepan with the hot milk mixture and reduce the heat to medium, continuing to cook while whisking constantly, until thickened and a few bubbled burst on the surface, about 30 seconds.

Remove the pastry cream from the heat and whisk in the butter and vanilla, then transfer to a bowl and cover with a sheet of plastic wrap placed directly onto the surface of the pastry cream so a skin doesn't form. Refrigerate until chilled and firm, about 3-4 hours.

Place the chocolate chips in a medium-size bowl. Heat the cream in a separate microwave-safe bowl for 60 seconds until hot, then pour over the chocolate chips. Let them sit for 2 minutes, then stir until the chocolate has melted completely into the cream. If necessary, you can microwave it for another 15 to 30 seconds to get the chocolate to melt all the way.

Dip the top half of each eclair into the chocolate ganache, then let them sit on a baking sheet or wire rack while the chocolate firms up a bit before serving.