Heat oven to 425 F. Coat six 6-ounce custard cups with nonstick spray.
Chop chocolates and melt along with butter in the top of a double boiler over simmering water or microwave chocolates and butter in a glass bowl, heating in 30-second intervals, until melted. Stir to combine.
Add flour and sugar to chocolate mixture. Lightly beat eggs and egg yolks, then stir into flour-sugar mixture until smooth. Stir in vanilla and liqueur. Divide batter evenly among custard cups.
Bake at 425 F for 14 minutes. Edges should be firm but the center will be runny. Run a knife around the edge to loosen and invert onto dessert plate. Makes 6 individual cakes.