Make the Choux Pastry
Preheat oven to 400°F (200°C). In a saucepan, heat butter, water, sugar, and salt until it boils. Remove from heat and stir in flour all at once. Return to heat and stir vigorously until the dough pulls away from the sides and forms a smooth ball (about 2 minutes). Let it cool for 5–10 minutes.

Add the Eggs
Beat in the eggs one at a time, mixing well after each. The dough should be smooth, glossy, and pipeable. Transfer to a piping bag with a large round tip and pipe 4–5 inch lines onto a parchment-lined baking sheet. Bake 25–30 minutes until puffed and golden. Cool completely.

Make the Pastry Cream
In a saucepan, whisk milk, sugar, cornstarch, and yolks over medium heat. Stir constantly until thick and bubbly (about 5–7 minutes). Remove from heat, stir in butter and vanilla. Let cool completely, then chill.

Fill the Éclairs
Cut éclairs in half or poke a hole in the bottom. Pipe chilled pastry cream into each.

Make the Ganache & Glaze
Heat cream until just steaming, pour over chopped chocolate, and let sit for 2 minutes. Stir until smooth and glossy. Dip the tops of the éclairs into the ganache and let set.