Flour Free Chocolate Cake

Several weeks ago, I found this recipe in a Joanne Fluke mystery novel.  I enjoy listening to them while I knit.  They are not the really scarie ones, that would keep me awake at night, but they are fun to listen to. This cake has no flour, or expensive substitutes in it. It truly was a good cake and I will make it again. :)

Flourless Chocolate Cake
Oven 375 F
1/2 c butter
8 oz chocolate chips (I used 1 1/3 cups)
4 egg yolks
1/2 cup granulated sugar
1/2 tsp vanilla (or rum extract)
4 egg whites
1/4 cup granulated sugar
Toppings
-sweetened whipped cream, shaved chocolate, berries or even ice cream.

8 inch spring form pan(I have one and used it, but next time, I am going to use my 9 inch square pan.)
Grease pan, if using spring form - grease it and line the bottom with wax papper.
-Melt chocolate.
-Beat 1/2 cup sugar with egg yolks, until lemon colored.
-Add small amount of yolk mixture to chocolate, while stirring.
-Pour chocolate into yolks, stir.
-Beat egg whites, with remaining sugar, until soft peaks form.
-Fold chocolate/egg mixture into bowl with egg whites until uniform.
-Bake at 375 F, for 35 minutes or until pick inserted in center of cake comes out clean.
For spring form pan:
-Cool 15 minutes, loosen spring form edge and turn onto serving platter. 
-Remove wax paper, when cake is cooled.
For regular pan: Leave cake in pan and serve from pan.
-Serve with any or all of the above toppings, or make your own topping.  :)