Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper.
In a saucepan, bring water and butter to a boil. Add flour and stir until mixture forms a ball.
Remove from heat and let cool slightly. Beat in eggs one at a time until smooth.
Transfer dough to a piping bag and pipe onto baking sheet.
Bake for 20 minutes or until golden brown. Let cool.
For filling, heat milk in a saucepan until simmering. In a bowl, whisk sugar, cornstarch, and egg yolks.
Gradually add hot milk to egg mixture, then return to saucepan. Cook until thickened. Stir in butter and vanilla.
For topping, heat cream and pour over chocolate chips. Stir until smooth.
Fill eclairs with custard and dip tops in chocolate. Chill before serving.