Step 1Heat the oven to 325 degrees. Butter a 1 1/2-quart souffle dish and sprinkle with sugar.
Step 2Melt the chocolate with 2 tablespoons of the sugar and the hot water in a small saucepan over low heat, stirring occasionally, until the chocolate is smooth. Remove from the heat and set aside.
Step 3Melt the butter in a skillet. Add the flour and salt and cook over low heat, stirring constantly, 2 to 3 minutes. Gradually stir in the milk and bring just to a boil, stirring constantly. The sauce should be smooth and thick. Blend in the chocolate mixture.
Step 4Beat the egg yolks slightly. Stir 1 to 2 tablespoons of the hot chocolate sauce into the yolks, then add the yolk mixture to the sauce in the skillet. Stir well, remove from the heat and set aside to cool.
Step 5Beat the egg whites until foamy. Gradually add the remaining 3 tablespoons of sugar and continue beating until the whites are stiff but not dry. Stir 1/4 of the whites into the chocolate mixture, then fold in the remaining whites. Stir in the vanilla.
Step 6Spoon into the souffle dish. Bake until the souffle is very puffed, 35 to 40 minutes.