Place 5 1/2 ounces of the chocolate and the butter in a metal bowl, and set it over a pan of simmering water. Stir until melted and smooth. Cool slightly.
Using a standard electric mixer with the whisk attachment, beat the eggs, yolks and sugar at medium-high speed until they are pale and thick, about 10 minutes.
Reduce the speed, and gradually mix in the flour.
Add the chocolate mixture to the flour mixture, and continue to beat until thick and glossy, about 5 minutes.
Divide half of the mixture among the prepared custard cups. Divide the remaining 2 ounces of coarsely chopped chocolate among them, placing the chunks in the very center of the chocolate mixture (there should be about 1 tablespoon of chopped chocolate per custard cup). Then top with the rest of the chocolate batter.
Bake until the cake is set around the edges but the center jiggles slightly when the cup is moved, about 12 minutes. Cool slightly (no more than 5 minutes).
Run a sharp knife around the edges of the cups. Turn the cakes out onto plates, and serve.
Recipe courtesy of David Rosengarten, Taste, Random House, 1998