Preheat oven to 350° with rack in the center.
TO PREPARE BAKING PANS: Cut two 9-inch parchment paper circles for bottom of baking pans, and two 4 x 30-inch parchment strips for sides of pan. Spray bottoms and sides of two 9-inch round cake pans with non-stick cooking spray. Place parchment paper circles on bottoms, and position strips around sides of pans. Spray pans again to coat the parchment paper lining.
TO MAKE BATTER: Melt butter in a small bowl, then mix in cocoa powder to make chocolate sauce. Set aside in a warm place so chocolate sauce stays melted.
In a large bowl, beat eggs. Add oil and milk or water and mix again. Add the bags of Vanilla Cake Mix and mix until batter is smooth. Remove 2 cups of the batter and stir them into the chocolate sauce until completely combined with a uniform color; set aside.
Split vanilla batter evenly between the two prepared cake pans, scraping the sides of the bowl with a rubber spatula to use all the batter. Tap filled pans on the counter once or twice to release air bubbles. Batter should look even in pan.
TO MARBLEIZE THE CAKE BATTER: First drop five tablespoon-size dollops of chocolate batter evenly around the top of the vanilla batter in each of the cake pans. Add any remaining chocolate batter evenly to the original dollops until the batter is used up.
Next, swirl the chocolate batter with the vanilla batter using a plain dinner knife, butter knife, or small offset spatula. Starting with the chocolate dollop in front of you, use the knife to draw a circle through the chocolate into the vanilla batter next to it two or three times to make streaks in the batter; be careful not to mix too much. Move on to the next chocolate dollop and repeat until you have made it all the way around both of the pans. Where large chocolate spots remain, run a knife through them, back and forth across the pan to make streaks in the center.
Bake for 47 to 53 minutes. To check for doneness, insert toothpick into center of chocolate cake (the chocolate cake takes slightly longer to cook); toothpick should come out clean or with just a few crumbs. Set pans on wire rack to cool completely.
TO MAKE THE FROSTING: Sift the Dark Chocolate Frosting Mix into the bowl of an electric stand mixer with paddle attachment. Add the butter and mix on medium speed until completely smooth. Add vanilla extract and mix until incorporated.
TO FROST THE CAKE: Remove one cake from pan and remove parchment from cake. Use a long serrated knife to trim rounded top off of cake so that it is flat. Put cut side down on cake plate so crumbs don’t coat your knife as your frost cake. Using half of the frosting, frost only the top of the layer.
Remove second cake from pan and trim like the first layer. Put cut side down on top of frosted first layer. Use the remaining frosting to frost the top of the cake just to the edge, letting the sides of the cake show.
NOTE: Chill a little before serving; the frosting will be firmer, making it easier to cut.
© Pamela's Products, Inc.