Preheat oven to 350 F. Combine cocoa, oil and bananas in a food processor or blender. Puree until smooth. Combine sweetener, eggs, flour and banana mixture in a mixing bowl. Beat until blended, then stir in the nuts. Pour the batter into a lightly oiled 9 x 13 inch baking pan and bake for 15-20 minutes or until done. Toothpick inserted into center should come out clean. (My suggestion - line baking pan with waxed paper and spray with non-stick vegetable spray.) Cut into 2 1/4 inch squares when cool.