Heat the oven to 392°F (200 °C). Butter 4 souffle forms and sprinkle them with sugar.
Return the mixture to the pot and stir on low heat until it thickens. Right before it comes to a boil, remove from the stove and leave it to cool. Once it cools, add the left over egg yolk.
Beat the 4 egg whites with a pinch of salt. Carefully add the whites to the mixture. Fill the forms and bake the souffles for 20 min. Take them out and sprinkle with powdered sugar.