Preheat oven to 350°F.
In a mixing bowl, sift together flour, sugar, cocoa, baking soda and salt. Add vanilla, mayonnaise and water. Beat until smooth.
Pour batter into a greased and floured 13x9-inch pan.
Bake until a toothpick inserted in center of cake comes out clean (about 30-35 minutes).
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Chocolate Mayonaise Cake recipe