Preheat oven to 350 degrees F. Butter and flour 6 (6-ounce) ramekins.
Place 5 1/2 ounces of the chocolate and the butter in the top of a double boiler, and melt them over simmering water.
Using a mixer with a whisk attachment, beat the eggs, egg yolks and sugar together until pale and thick, about 8 minutes. Reduce speed and add flour. Add the chocolate mixture and continue beating until glossy, about another 5 minutes.
Divide half the mixture into the ramekins. Divide the remaining chopped chocolate among the ramekins, making a little pile in the middle of each one. Top with the rest of the batter.
Bake until just set around the edges, but the center still jiggles. About 12 minutes. DO NOT overcook.
Cool just a few minutes and unmold each cake onto a plate.