1.Make crème pâtissière. Combine milk, sugar and extract in a medium saucepan; bring to the boil. Meanwhile, whisk egg yolks and cornflour in a medium heatproof bowl; gradually whisk in hot milk mixture. Return mixture to pan; stir over medium heat until mixture boils and thickens. Cover surface with plastic wrap. Refrigerate 4 hours.
2.Preheat oven to 220°C/425°F. Grease oven trays.
3.To make choux pastry, combine the water, butter and caster sugar in a medium saucepan; bring to the boil. Add flour; beat with a wooden spoon over medium heat until the mixture comes away from base of pan. Transfer pastry to a medium bowl; beat in two of the eggs, one at a time. Whisk remaining egg with a fork; beat enough of the egg into the pastry until it becomes smooth and glossy but still holds its shape.
4.Drop level tablespoons of the pastry, about 5cm (2 inches) apart, onto trays.
5.Bake puffs for 10 minutes. Reduce oven temperature to 180°C/350°F; bake for a further 15 minutes. Cut a small opening into the base of each puff; bake for a further 10 minutes or until puffs are dry. Cool on trays.
6.Spoon crème pâtissière into piping bag fitted with a 5mm (¼-inch) plain tube; pipe through cuts into puffs. Dust puffs with sifted icing sugar.
Crème pâtissière can be made a day ahead; store, covered, in the refrigerator. It will need to be softened before piping: beat it with an electric mixer until smooth.Note