1. Preheat the oven to 200°C (400°F)
2. In the microwave, melt the chocolate and butter for 2 minutes at 500W (break the chocolate bar into squares, put the pieces in a bowl, add 2 tablespoons of cold water and melt for 1 minute, mix and add 30 to 60 seconds if necessary).
3. In a bowl, whisk the egg yolks and sugar.
4. Add the melted chocolate.
5. Beat the egg whites until stiff peaks form, then gently fold them into the mixture.
6. Pour the mixture into buttered and floured ramekins.
Bake in the oven for about 7 to 10 minutes, depending on the size of the ramekins (the cakes should be completely molten in the center).