Preheat the oven to 425 degrees F. Spray four custard cups with baking spray and place on a baking sheet.
Microwave butter, bittersweet chocolate and semisweet chocolate in a large bowl on high until the butter is melted, about 1 minute. Whisk until the chocolate is melted. Stir in the sugar until well blended. Whisk in eggs and egg yolks, then add the vanilla. Stir in the flour. Divide the mixture among the custard cups.
Bake until the sides are firm and the centers are soft, about 13 minutes. Let stand 1 minute.
Optional: Sprinkle some confectionary sugar on top before you serve