Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.

Boil water and butter in a medium saucepan over medium heat until it reaches a rolling boil.

Mix in the flour and salt, stirring vigorously until the dough forms a ball.

Cool the dough briefly in a mixing bowl, then add the eggs one at a time, mixing well after each addition.

Pipe the choux pastry into small strips (about 2 inches long) onto the parchment-lined baking sheet.

Bake for 20-25 minutes or until the éclairs are golden brown and puffed.

Cool the baked éclairs completely on a wire rack.

Heat the milk in a saucepan over medium heat until it just starts to simmer.

Whisk together the sugar, cornstarch, and egg yolks in a separate bowl until smooth.

Combine the hot milk with the egg mixture slowly while whisking constantly. Return this mixture to the saucepan and cook for 2-3 minutes.

Remove from heat, then stir in the vanilla extract and butter until smooth. Let this filling cool completely.

Slit the side of each cooled éclair using a small knife, then fill with the cooled pastry cream.

Simmer heavy cream in a small saucepan until it begins to bubble. Remove from heat and add chocolate chips.

Dip the top of each filled éclair into the chocolate ganache, allowing excess to drip off.

Set the dipped éclairs on a wire rack to allow the chocolate to firm up before serving.