Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
In a saucepan, bring water, butter, sugar, and salt to a boil. Remove from heat and stir in flour until it forms a ball.
Transfer dough to a mixing bowl. Add eggs one at a time, beating well after each addition.
Pipe dough onto the baking sheet in 4-inch strips. Bake for 20-25 minutes until golden brown. Let cool.
For the custard, heat milk in a saucepan. In a bowl, whisk sugar, cornstarch, and egg yolks. Gradually add hot milk, then return to saucepan and cook until thickened. Stir in butter and vanilla. Let cool.
Fill eclairs with custard using a piping bag.
For the glaze, heat cream and pour over chocolate. Stir until smooth. Dip tops of eclairs in glaze.