Step 1: Prepare the Choux Pastry

1. Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
2. In a medium saucepan, combine the water and butter. Bring to a boil over medium heat.
3. Remove from heat and stir in the flour and salt until a smooth dough forms.
4. Let the dough cool for a few minutes, then add the eggs one at a time, mixing well after each addition until the dough is smooth and glossy.
5. Transfer the dough to a piping bag fitted with a large round tip. Pipe long strips (about 4 inches) onto the prepared baking sheet, leaving space between each éclair.
6. Bake in the preheated oven for 25–30 minutes until golden brown and puffed. Do not open the oven door during baking. Remove from the oven and let cool completely on a wire rack.

Step 2: Make the Pastry Cream

1. In a saucepan, heat the milk over medium heat until it begins to steam (but not boil).
2. In a separate bowl, whisk together the sugar, cornstarch, and egg yolks until smooth.
3. Slowly pour the hot milk into the egg mixture while whisking constantly to temper the eggs.
4. Return the mixture to the saucepan and cook over medium heat, whisking constantly until it thickens and bubbles. Remove from heat and stir in the vanilla extract and butter.
5. Transfer the pastry cream to a bowl, cover with plastic wrap pressed directly on the surface, and let it cool completely.

Step 3: Fill the Éclairs

1. Once the choux pastries are cool, use a sharp knife to make a small slit in the side of each éclair.
2. Fill a piping bag with the cooled pastry cream and pipe it into each éclair through the slit until filled.

Step 4: Make the Chocolate Glaze

1. In a small saucepan, heat the heavy cream over medium heat until it begins to simmer.
2. Remove from heat and add the chopped chocolate and butter. Let sit for a minute, then stir until smooth and glossy.

Step 5: Assemble and Serve

1. Dip the top of each filled éclair into the chocolate glaze or drizzle it over the éclairs.
2. Place the éclairs in the refrigerator for about 30 minutes to set the glaze. Serve chilled or at room temperature.