Chop cranberries finely (eg by cuisinart) and add 1/2 c sugar.
Let sit for at least 1/2 hour.
Heat oven to 330 degrees on convection/bake
(350 degree in a conventional oven)
Melt chocolate with butter.
Butter a 9x9 inch baking dish.
Beat 1/4 cup sugar with egg yolks until the mixture turns thick and pale.
Add cranberries and mix well.
Add chocolate/butter mixture and mix well.
Add flour and mix gently.
Beat egg whites with cream of tartar until soft peaks are formed.
Beat 1/4 cup sugar into egg whites until stiff peaks are formed.
Gently fold the beaten egg whites into the batter.
Pour the batter into the baking dish.
(gently stir in the optional chocolate).
Bake 30 minutes or until a toothpick comes out barely moist.
(33-35 minutes in a conventional oven)