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 forms and pulls from sides.
Cool slightly and 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.
Heat milk for pastry cream, whisk yolks, sugar, and cornstarch, slowly combine, cook until thick, stir in butter and vanilla, and chill.
Pipe chilled pastry cream into cooled 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.
