Generously butter eight ¾-cup ramekins. Sprinkle inside of each with 1½ tsp sugar.
Stir chocolate and butter in heavy medium saucepan over low heat until smooth.
Using electric mixer, beat eggs, egg yolks, and remaining 5 tbsp sugar in large bowl until thick and pale yellow, about 8 minutes.
Fold 1/3 of warm chocolate mixture into egg mixture, then fold in remaining chocolate. Fold in flour.
Divide batter among ramekins. (Can be made 1 day ahead. Cover with plastic, chill. Bring to room temp before continuing.)
Preheat oven to 425°F. Place ramekins on baking sheet. Bake uncovered until edges are puffed and slightly cracked but center still moves slightly, about 13 minutes.
Top each cake with a scoop of vanilla bean ice cream and serve immediately.