Preheat oven to 350 degrees F (175 degrees C). Grease one 11x7 inch pan.
Sift together flour, sugar, cocoa, baking powder, and soda. Add mayonnaise, cold water, and vanilla. Beat 2 minutes. Pour into prepared pan.
Bake at 350 degrees F (175 degrees C), 30-35 minutes. Frost if desired.
