Eating this scrumptious yet simple dessert will always remind me of Paris, where I enjoyed my first "Moelleux au Chocolat" — fortunately for me and all other chocolate lovers this dish transfers very well to the states.
In order to get this chocolate cake to rise (like a soufflé), the eggs need to be whipped, so that the air pockets inside them can expand in the oven. Long story short, when I first made these, my mixer broke so I had to whip the eggs by hand — it is totally doable, but you may get a severe hand cramp afterward. If you have an electric mixer, I advise using it.
Preheat the oven to 375 degrees.
Bring a small saucepan of water to a boil, then reduce to a simmer. Place the chocolate and butter in a heatproof bowl (and one that is bigger than the rim of the saucepan), and place over the saucepan. Gently stir until the butter and chocolate are combined. Remove from heat.
In a small bowl, beat the egg with a fork until the yolk and white are fully combined. Pour half of the mixture into another bowl.
Add the sugar to one of the eggs bowls - discard the egg in the other bowl. Beat until a light, canary-yellow color has formed, and the mixture has tiny bubbles on the surface. (If using an electric hand-held mixer, this should take about 3-4 minutes on medium speed).
Add the flour to the eggs and gently stir to combine. Pour in the chocolate mixture and gently fold into the eggs and flour, until just combined.
Butter a 6-ounce ramekin, using an upward vertical motion when greasing, to help the soufflé rise. Pour in the mixture, and bake in the oven until the outside is firm and the center is still wobbly, about 12 minutes.
Dig in! This can be served with a dollop of whipped cream, crème fraiche, or a scoop of vanilla ice cream.