1. Preheat oven to 425°.
2. Place a saucepan of water on high heat. Heat to a boil, then reduce to simmer.
3. In a mixing bowl, crack eggs and warm them over simmering water for 1 minute (hold bowl directly over the water, using a dish towel to protect your hands), whisking constantly.
4. Remove eggs from heat, and whisk for another minute until eggs are very fluffy. Meanwhile, put the chocolate and butter in a large, heatproof, stainless-steel or tempered-glass bowl; set it over the simmering water, making sure the water doesn't touch the bowl. Let the chocolate and butter melt; remove from heat. Let mixture cool slightly by stirring occasionally, about 5 minutes.
5. Once slightly cooled, fold the chocolate and butter into the whipped eggs until no more streaks show. Batter will deflate.
6. Spray 6 (4-ounce) ramekins or ovenproof dessert cups with nonstick cooking spray (or use foil cups, or even muffin tins). Fill them 3/4 of the way full and place on a baking sheet.
7. Bake for 10-12 minutes, depending on how gooey you'd like the centers. Turn the cakes out onto dessert plates, and serve immediately with a sprinkle of powdered sugar.