Make the choux pastry:
Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
In a saucepan, combine water and butter, and bring to a boil over medium-high heat. Once the butter has melted, remove from heat and stir in the flour and salt until the dough forms a ball.
Return to low heat and cook the dough, stirring constantly for about 2 minutes, until slightly dried out.
Remove from heat and cool slightly before adding eggs one at a time, mixing well after each addition. The dough should be smooth and slightly sticky.
Transfer to a piping bag and pipe 4-inch long lines, spacing them about 2 inches apart.
Bake for 20-25 minutes until puffed up and golden brown. Let cool completely on a wire rack.

Make the pastry cream:
In a saucepan, heat milk and vanilla (extract or bean) over medium heat until simmering.
In a separate bowl, whisk egg yolks, sugar, cornstarch, and salt until smooth.
Gradually add the hot milk to the egg mixture, whisking constantly to avoid curdling.
Return the mixture to the saucepan and cook over medium heat, whisking until thickened and boiling.
Stir in butter until smooth and transfer to a bowl. Cover with plastic wrap (pressing directly onto the surface) and chill for at least 2 hours.

Make the chocolate glaze:
In a small saucepan, heat heavy cream until it begins to simmer.
Pour the hot cream over the chopped chocolate and let sit for 1-2 minutes. Stir until smooth and glossy. Stir in butter.

Assemble the éclairs:
Once the éclairs have cooled and the pastry cream is chilled, make a small slit in the side of each éclair.
Fill the éclairs with the chilled pastry cream using a piping bag.
Dip each éclair into the chocolate glaze, allowing excess glaze to drip off. Let the glaze set before serving.