Preheat the oven to 200°C.
Cut the butter into cubes and add to a saucepan. Crush up the sugar cubes and add them, the water and the salt to the pan and bring to the boil. Remove from the heat and then stir in the flour. Return to the heat and stir constantly so that the mixture does not stick to the bottom of the pan.
Pour the mixture into a bowl and then mix in the eggs one at a time. Mix well and then transfer to a piping bag. Pipe the mixture into small balls on a lined baking tray. Brush each chouquette bun with beaten egg yolk and sprinkle with sugar crystals.
Transfer to the oven and bake for 5 minutes before lowering the heat to 160°C. Finish cooking for 25 minutes.