Melt chocolate over a double-broiler until completely melted.
To make the meringue, beat the egg whites until frothy. Add cream of tartar and continue beating until opaque.
Slowly add in the sugar and continue beating until stiff peaks.
Whisk in the egg yolks in the melted chocolate.
Fold in one-third of the meringue into the chocolate. Gently fold the remaining thirds of the meringue, one at a time.
Brush the inside of the ramekins with butter and sprinkle powdered sugar.
Spoon the batter into the prepared ramekins, filling them up to the top. Bake at 400F for 10-12 minutes.