1.To make pastry cream: Bring milk to the boil in a medium saucepan. Beat egg yolks, sugar and cornflour in a small bowl with an electric mixer until thick. With motor operating, gradually beat in hot milk, then vanilla. Return custard mixture to clean saucepan; whisk over medium heat until mixture boils and thickens. Remove from heat, transfer to a medium bowl; cover surface with plastic wrap. Cool, stirring occasionally. Refrigerate.
2.Meanwhile, preheat the oven to 180°C(l60°C fan- forced). Line two oven trays with baking paper.
3.Combine butter and the water in a medium saucepan; bring to the boil over medium heat. Add flour all at once; stir vigorously over heat until the mixture leaves the side of the pan and forms a smooth ball. Transfer mixture to the small bowl of an electric mixer. Beat in eggs, one at a time, until mixture is smooth and glossy.
4.Spoon mixture into a large piping bag fitted with a 1.5cm plain tube; cool. Pipe 20 x 7cm lengths of pastry, about 5cm apart, on prepared trays. Bake for about 25 minutes or until browned lightly and crisp; cool on wire racks.
5.To make chocolate glaze: Stir butter and the water in a small saucepan over a low heat until the butter is melted. Add the chocolate, stir until melted. Remove from heat; stir until smooth and shiny. Stand until thickened slightly.
6.Split the eclairs in half. Spoon chocolate glaze over the tops of the eclairs, spread with the back of a spoon; stand until set.
7.Transfer the pastry cream to a piping bag fitted with a small plain tube. Fill eclairs with the pastry cream just before serving. Replace tops.
Unfilled eclairs suitable to freeze. Glaze suitable to microwave.Note