Coarsely chop chocolate squares. In a medium bowl, microwave chocolate and butter in 15-second intervals. Stir between intervals with a rubber spatula until the chocolate is melted.
Add sugar and stir until dissolved.
In a small bowl, whisk eggs with egg yolks.
Add eggs and vanilla to the chocolate and sugar mixture. Fold in flour just until the flour is no longer visible.
Divide the batter evenly among the custard cups and place the cups on a baking sheet.
Bake for about 12 minutes. The edges of each cake should be firm, but the center will be soft.
Remove from the oven and let sit on the counter for about 3 minutes. Run a knife around the edges to loosen, then invert cakes onto dessert plates.
Serve immediately.
Notes
Use good quality baking chocolate. I like Ghirardelli bittersweet, sold in the baking aisle. I also use Ghirardelli or Guittard chocolate chips.Make sure to generously grease the cups or you will have a hard time removing the cakes in one piece.I tested this recipe twice, found the following: It yields 6 large lava cakes for people who really like a rich, good-sized portion of dessert. If you are serving this with ice cream and berries, divide the batter into 8 cups.If baking ahead, bake the cakes for about 10 minutes, let sit for a few minutes, run the knife around the edges and then let cool. When ready to serve, invert and microwave for about 15 seconds or leave in custard cups, cover with foil, and warm in the oven at 275 degrees for about 10 minutes.