Prepare cake mix according to package directions, stir in sour cream. Divide batter into 2 medium bowls. Stir graham cracker crumbs into one bowl and baking cocoa into the other. Set aside.
Pour graham cracker cake batter into a greased 13x9-inch pan. Top with chocolate cake batter; swirl together. Top with marshmallows.
Bake in a preheated 375°F oven 28-30 minutes or until knife inserted in center comes out clean.
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