Preheat oven to 350 degrees F (180 degrees C). Line an a large baking tray with baking paper.
Put the water, milk and butter into a small saucepan. Bring the mixture the boil and then lower the temperate to a simmer, sift in the flour and stir until the mixture is smooth. Take the saucepan off the stove. Whisk in the eggs, one at a time, mixing well after each egg is added. Add the sugar and combine well.
Put the dough in a piping bag with a large nozzle (or a large plastic bag with the corner cut out). Add ball about 2 inch in diameter (5cm). to the baking tray. If you keep the nozzle inside each ball as it is being made and squeeze you will made largish round balls, rather than flat ones. The rounder and smoother the better.
Bake the try of balls in the preheated oven for 30-40 minutes, until the balls are puffed and golden brown in color. Poke a hole in the side of each ball while they are still hot on the baking tray. You can use a chopstick or similar implement for this. Making the holes when the balls are hot, stops them shattering.
Return the tray of balls to the oven, and switch it off. Leave the tray in the oven while it cools. This helps to dry the balls, include the inside. This works because of the holes that have been made in each ball.
Add the icing sugar to the cream and whip until firm peaks form. Spoon the cream into a a piping bag with a small nozzle that will fit into the holes you made in the balls. Fill the inside of each ball with the cream mixture.
Th make the chocolate sauce, melt the chocolate on a bowl suspended over gently steaming saucepan of water. Stir until the chocolate has melted.
Hold each ball filled with cream at the bottom and dip into the melted chocolate to coat the top of each base. Cool the balls on a wire rack. Chill and serve.