These cakes are meant to have a hot, flowing chocolate center that oozes out when you cut into them. Oven temperature and baking time are critical to the success of the recipe. Be sure to check at the minimum bake time. If the cakes are too soft or do not hold their shape, bake a minute or two longer. If the centers are too cakelike in texture, bake a few minutes less next time.
Make the batter a day ahead and pour into custard cups. Cover with plastic wrap and refrigerate up to 24 hours. You may need to bake the cakes 1 to 2 minutes longer.