Preheat oven to 350 degrees F. Grease a 9x9 inch baking pan.
Whisk dry ingredients together in medium mixing bowl. Add the water, mayonnaise, and vanilla and mix until evenly combined. Pour batter into prepared baking pan. Bake for 25-30 minutes or until toothpick comes out clean.
For the Fudge Sauce:
Place chocolate chips in a bowl. Heat whipping cream, pour over chocolate chips, and cover. Let sit for 5 minutes and then whisk until smooth.
Serve cake warm or reheat. Spoon fudge sauce on top of cake, sprinkle with walnuts, top with ice cream. Serve immediately.