Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper to prevent sticking and ensure easy cleanup later.

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

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

Cool the dough for a few minutes, then add the eggs one at a time, mixing well after each addition until smooth.

Pipe the dough into long strips (about 4 inches) onto the prepared baking sheet.

Bake in the preheated oven for 25–30 minutes until golden brown and puffed.

Remove from the oven and let cool completely on a wire rack.

Heat the milk in a saucepan over medium heat until it begins to steam.

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

Temper the egg mixture by slowly pouring the steaming milk into it while whisking continuously.

Return the mixture to the saucepan and cook over medium heat, whisking constantly until it thickens.

Remove from heat and stir in the vanilla extract and butter.

Cool the pastry cream in a bowl with plastic wrap pressed directly on the surface.

Make a small slit in the side of each cooled éclair using a sharp knife.

Fill a piping bag with the cooled pastry cream and pipe it into each éclair.

Simmer the heavy cream in a small saucepan over medium heat until it just begins to bubble.

Remove from heat and add the chopped chocolate and butter, stirring until smooth.

Dip the tops of each filled éclair into the chocolate glaze.

Chill the éclairs in the refrigerator for about 30 minutes to allow the glaze to set.