Preheat the oven to 190 degrees C. Butter six ramekins and dust with white sugar. Place on a baking tray.
Combine the chocolate and butter in a bowl over a saucepan of simmering water and heat, stirring, until chocolate has melted. Remove from heat and whisk in half the sugar. Whisk in the egg yolks.
With an electric mixer, beat the egg whites with a pinch of salt until soft peaks form. Add the cream of tartar and beat a little more. Gradually add remaining sugar and beat until stiff peaks form. Fold the egg whites into the chocolate mixture and divide between the ramekins.
Bake 15-20 minutes, until cracked on top. Note: a skewer will not come out clean, they are meant to be slightly molten.