Preheat oven to 325 °F. Butter and flour the 6 ounce custard cups. Mix chocolate and butter in metal bowl. Set over saucepan of simmering water; stir until smooth. Cool slightly.
With electric mixer, beat eggs, yolks and sugar until pale and thick, about 10 minutes. Reduce speed, gradually beat in flour. Add chocolate mixture and beat until thick and glossy, about 5 minutes. Pour into prepared cups.
Bake until cake is set around edges but center moves slightly when cup is moved, about 12 minutes. Cool slightly. Run sharp knife around edge of cakes. turn out onto plates. Serve with ice cream. Makes 6 individual cakes.