Preheat oven to 325°F. Prepare 12 x 18 in. pan and heating core with vegetable pan spray.

In large bowl, mix 1 cake mix, 1/2 cup melted butter, 1 cup milk, 3 eggs and 2 teaspoons vanilla extract for 30 seconds on low speed. Scrape bowl and beat on medium speed for 2 minutes. Pour into prepared pan. Repeat with second and third cake mixes; reserving 3 cups of yellow cake batter. Place heating core in center of pan and fill half full with batter. Stir cocoa powder into reserved batter until smooth. Drizzle chocolate batter over yellow batter; swirl with knife.

Bake 48-52 minutes or until toothpick inserted in center comes out clean. Cool in pan on cooling grid 10 minutes. Remove from pan; cool completely on grid. Ice cake with chocolate buttercream.