For sauce: Stir semisweet and unsweetened chocolates in top of double boiler over barely simmering water just until melted. Add 1/3 cup hot water, Karo Syrup and extract; whisk until smooth. Remove from double boiler and cool slightly.
For cakes: Preheat oven to 450°F. Butter six 3/4-cup ovenproof soufflé dishes or custard cups- such as Pyrex. Stir chocolate and butter in heavy saucepan over low heat until melted. Cool slightly. Whisk eggs and egg yolks in large bowl to blend. Whisk in sugar, then chocolate mixture (a little at a time to avoid scrambling the eggs) and flour. Pour batter into dishes, dividing equally.
Bake cakes until sides are set but center remains soft and runny, about 11 minutes. Run small knife around cakes to loosen. Immediately turn cakes out onto plates. Spoon sauce around cakes. Serve with ice cream.