Butter six 1-cup ramekins (4 by 2 inches) and coat with sugar, knocking out excess sugar.
Finely chop chocolate. In a small saucepan melt butter over low heat. Add cream and bring just to a boil. Remove pan from heat and add chocolate, stirring until smooth. Transfer mixture to a large bowl and stir in yolks.
In another large bowl with an electric mixer beat whites with cream of tartar and a pinch of salt until they just hold stiff peaks. Gradually add 1/4 cup sugar, beating until just combined. Stir one fourth whites into chocolate mixture to lighten and fold in remaining whites gently but thoroughly.
Divide souffle mixture among ramekins and smooth tops with a knife. Run tip of knife around edges of souffles to aid raising. Souffles may be made up to this point 1-day ahead and chilled, loosely wrapped in plastic wrap.
Preheat oven to 375 degrees F.
Bake souffles on a baking sheet in lower third of oven until puffed and surfaces are cracked, about 20 minutes. Top souffles with sauce and serve immediately.
In a heavy-bottomed saucepan, simmer sugar, water, corn syrup, and a pinch of salt, stirring until sugar is dissolved. Boil mixture, without stirring, until a golden caramel. Remove pan from heat and add cream and vanilla, stirring until combined well, about 1 minute. Cool sauce at room temperature (sauce will thicken as it cools). Sauce keeps, covered and chilled, 3 weeks. Bring sauce to room temperature before serving.
Yield: 3/4 cup