Put the water to boil with the butter and salt. Then pour the flour in all at once and stir with a wooden spatula until the dough becomes one that separates easily.
Allow to cool and mix in the eggs, one at a time, until each is absorbed separately. Pour the dough into a pastry bag or icing piping syringe and spray out eclairs in a tray covered with baking paper.
Put to bake in a moderately preheated oven until doubled in size and slightly reddish. Do not open the oven door during baking, because it can ruin the eclairs. Once ready, leave to cool inside with the oven switched off.
For the cream mixture, melt the chocolate with the butter and sugar in a water bath without boiling. Allow the chocolate mixture to cool. Beat the liquid cream separately, then gently stir and gradually add the chocolate.
Fill the eclairs with the resulting cream mixture. Optionally you can set aside a little of the chocolate with butter to make decorations on top of the ready eclairs.