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Preheat oven to 375°.
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Sift together into large bowl, flour, 1 1/4 cups sugar, baking powder and salt.
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In another bowl beat yolks, 3/4 cups water, oil and vanilla on high speed until smooth.
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Stir into the flour mixture until smooth.
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In another clean, dry bowl beat egg whites and cream of tartar on medium speed until soft peaks form.
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Gradually add 1/4 cups sugar. Beat until peaks are stiff but not dry.
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Using a rubber spatula, fold 1/4 of the egg whites into the yolk mixture, then add remaining whites. Do not Stir!
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Pour into 9 x 13 pan.
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In a measuring cup with a pourable lip, mix cocoa with remaining water and sugar.
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Slowly pour over cake in a grid like pattern. Using a fork, cut through the batter to marbelize, following the pattern of the cocoa.
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Immediately place in oven for 1 hour.