1. Put the water and butter in a small saucepan and heat gently until the butter has melted. Bring to the boil, remove the pan from the heat and immediately beat in the flour and salt until evenly combined.
2. Return the pan to a gentle heat and cook, stirring with a wooden spoon, until the mixture comes together and starts to leave the sides of the pan.
3. Remove from the heat and beat in the eggs with a wooden spoon a little at a time, beating well after each addition until the egg is incorporated and the mixture is smooth and shiny. Use as required.