In a saucepan, bring the water and milk to boil with the butter, sugar and salt.
Remove from heat and mix with the flour.
Stir with a spatula until the mixture forms a thick dough that doesn’t stick.
Put the pan back on a low heat and dry the dough for 20 seconds and keep stirring away from heat for another 3 min.
Remove the pan from the heat and add each egg, one after the other, mixing vigorously until the egg and the batter are assimilated. The batter needs to becomes runny but not liquid.
Put the last egg gradually because too much egg would make the batter too liquid.
Know if you have the good texture, pass a tea spoon in the dough, if it close in less than 1 second it is too runny, if it doesn’t move it needs a bit more egg. With the right texture it takes around 5 second.
Put the batter in a pipping bag with a medium round nozzle tip.
Pre-heat the oven 170C.
Pipe 10cm/4in lengths onto the greased baking tray.
Glaze with the egg yolk mix with a tea spoon of water, the glaze help removing the spike left by the piping.
Bake for about 25 minutes. The puff have to dry or else they will flatten as soon as you take them out.
Make 2 holes at the back of your puff.
Put the Chocolate Custard in a pipping bag with a small and pointy round nozzle tip.
Pipe in the custard both side of each puff.
Prepare the ingredients for the icing.
Take the sugar, glucose and water to boil until 115C (around 10min).
Take away from heat and pour it in another container or else it will continue to harden.
Add quickly the chocolate powder and keep stirring for 5 min.
Without burning your finger, dip the tops in the icing, remove the extra that might run on the sides and let it set before serving.