Melt the butter in a small saucepan; stir into it the flour and let it cook a minute, but not brown, then add slowly the milk and stir until smooth and a little thickened; remove it from the fire and turn it slowly onto the yolks and sugar, which have been beaten to a cream; mix thoroughly and add the melted chocolate; stir for a few minutes, then set it away to cool; rub a little butter over the top so a crust will not form. When ready to serve, stir the mixture well to make it smooth and fold into it lightly the whites of the eggs, which have been whipped until very dry and firm. Turn the mixture into a buttered tin, filling it two thirds full. Have the tin lined with a strip of greased paper which rises above the sides to confine the souffle as it rises. Place the tin in a deep saucepan containing enough hot water to cover one half the tin. Cover the saucepan and place it where the water will simmer for thirty minutes, keeping it covered all the time. Place the tin on a very hot dish and serve at once. Cover the top with a hot tin until it reaches the dining-room if it has to be carried far.
If unsweetened chocolate is used, add about three more tablespoonfuls of sugar or to taste, and a teaspoonful of vanilla.