Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a saucepan, heat butter, water, sugar, and salt until boiling.
Stir in flour quickly and cook 1–2 minutes until dough pulls from sides.
Cool slightly, then beat in eggs one at a time until smooth and glossy.
Pipe 4-inch strips onto baking sheet and bake 25–30 minutes until puffed and golden.
Cool shells completely before filling.
For pastry cream, heat milk until warm; whisk yolks, sugar, and cornstarch; slowly add milk; cook until thick; stir in butter and vanilla; chill.
Pipe chilled pastry cream into eclair shells.
Heat cream for glaze, pour over chopped chocolate and butter, stir until smooth.
Dip tops of filled eclairs in glaze and refrigerate until set.