Preheat Oven:

Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper.

Make Choux Pastry:

In a medium saucepan, combine water, butter, and salt. Bring to a boil over medium heat. Add flour all at once, stirring vigorously until dough forms a ball and pulls away from the sides of the pan. Remove from heat and let cool for 5 minutes.

Add Eggs:

Beat in eggs one at a time until fully incorporated and smooth.

Pipe Dough:

Transfer dough to a piping bag fitted with a large round tip. Pipe 4–5 inch long strips onto the prepared baking sheet, leaving space between each.

Bake Éclairs:

Bake at 425°F (220°C) for 10 minutes, then reduce heat to 350°F (175°C) and bake an additional 20–25 minutes until puffed and golden. Avoid opening the oven during baking. Cool completely on a wire rack.

Prepare Pastry Cream:

In a medium saucepan, heat milk until just simmering. In a bowl, whisk together sugar, cornstarch, and egg yolks until smooth. Slowly whisk hot milk into egg mixture, then return to saucepan. Cook over medium heat, whisking constantly, until thickened. Remove from heat, stir in butter and vanilla. Cool completely.

Fill Éclairs:

Slice cooled éclairs horizontally or poke a hole in the side. Pipe pastry cream into each éclair using a piping bag.

Make Chocolate Glaze:

Heat heavy cream until just simmering, pour over chopped chocolate, let sit 1–2 minutes, then stir until smooth.

Glaze Éclairs:

Dip the top of each filled éclair into the chocolate glaze. Allow glaze to set slightly before serving.