Melt chocolate and butter together in microwave or double boiler or straight in pot over stove, whichever is easy for you
Stir in flour a little at a time until mixed
Beat egg whites until stiff
Preheat oven to 350 F and grease a rectangular pan with butter lightly
Fold in beaten egg whites into chocolate mixture until combined well
Pour in melted butter mixture, mix well
Pour 1/2 batter into pan, sprinkle with chopped walnut then pout the rest of the batter and sprinkle top with more walnut
Bake for 25 minutes or until knife comes out clean
Great to be served warm or room temperature
Enjoy~