For the creme patisserie: Whisk egg yolks, sugar, vanilla, cornflour and flour until smooth. Add ¼ cup milk, whisk until combined. Stir in the remaining milk. Transfer to a saucepan. Cook, stirring constantly, over medium heat for 10 minutes or until custard just comes to the boil. Pour custard into a heatproof bowl. Cover the surface with wrap. Refrigerate until cold.
Preheat oven to 220°C. Melt butter in a saucepan over medium heat. Add water. Increase heat to high. Cover, bring to a rolling boil, stay over it so it doesn™t boil over as this will happen quickly, it should just begin to rise in the pan. Remove from heat. Quickly add all flour. Stir with a wooden spoon until mixture comes together and leaves side of saucepan.
Transfer hot mix to the bowl of an electric mixer. Turn mixer on medium-low, gradually add egg in 4 stages and beat until mixture is thick and shiny (you may have egg left over or may need a tiny more, this can vary, the mixture should be thick and have shine to it).
Lightly grease two baking trays, do not line them with baking paper. Half fill a piping bag (don™t fill more than halfway as too hard to control your piping). Cut about 4cm off the top of piping bag so the opening is about 11/2cm. Pipe 24 eclairs, each about 10cm long and 11/2cm wide, allowing room for spreading. Bake for 28-30 minutes or until choux pastry is puffed, golden and firm to the touch. Remove from oven and allow to cool completely on trays.