The most important rule to remember in making these individual cakes is to use not only the best chocolate you can find, but the chocolate you most love to eat, since that's the ingredient you will taste.
Usually these cakes are made in individual ramekins, but with all the ramekins stacked in my cupboard, I couldn't come up with six that were the right size. Then I discovered that disposable aluminum foil muffin pans are ideal. I buy the kind that have six cups and come two pans to a package. I also wash them in the dishwasher and re-use them.
Getting Ready: Center a rack in the oven and preheat the oven to 400 degrees F. Butter (or spray - it's easier) 6 cups of a regular-size muffin pan, preferably a disposable aluminum foil pan, dust the insides with flour and tap out the excess. Put the muffin pan on a baking sheet.
2. Set a heat proof bowl over a saucepan of gently simmering water, put the coarsely chopped chocolate and the butter in the bowl and stir occasionally over the simmering water just until they are melted - you don't want them to get so hot that the butter separates. Remove the bowl from the pan of water.
3. In a large bowl, whisk the eggs and yolk until homogenous. Add the sugar and whisk until well blended, about 2 minutes. Add the dry ingredients and, still using the whisk, stir (don't beat) them into the eggs. Little by little, and using a light hand, stir in the melted chocolate and butter. Divide the batter evenly among the muffin cups and sprinkle the finely chopped chocolate over the batter.
4. Bake the cakes for 13 minutes. Transfer them, still on the baking sheet, to a rack to cool for 3 minutes. (There is no way to test that these cakes are properly baked, because the inside remains liquid.)
5. Line a cutting board with a silicone baking mat or parchment or wax paper, and, after the 3-minute rest, unmold the cakes onto the board. Use a wide metal spatual to lift the cakes onto dessert plates.
Serving: These should be served as soon as they are put on plates. The cakes are not meant to be served alone - they need something to play off their warm, gooey, soooooo chocolaty interior. Ice cream is the most obvious choice and, to my mind, the best in terms of texture and, of course, temperature. Any chocolate-friendly flavor will be good. Circling the cakes with crème anglaise is another good idea and, for those for whom too much is not enough, circling the cakes with crème anglaise and running a ring of bittersweet chocolate sauce through the custard is an even better idea.
Storing: Although the whole point of a warm, runny cake is to eat it when it is warm and runny, the cake is still delicious, but different, the following day. If you wrap the cooled cakes in plastic wrap and keep them at room temperature, the next day the texture of the center of the cake (the part that was once gooey) will remind you of ganache. Eating the cake will be like enjoying a bonbon: it will be firm on the outside and creamy within.
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