Place the cake mix, eggs, mayonnaise, and water in a large bowl. Beat on medium speed for 2 minutes.
Pour into a greased 13x9 inch pan and bake for 30–35 minutes at 325°F.
Place the melted unsweetened chocolate, milk, butter, and vanilla in a large bowl with the powdered sugar.
Beat until light and fluffy.
Frost the cooled cake with the chocolate frosting. I like using an offset spatula to create a pretty swirled top.
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