I made this cake for Sam's 28th Birthday.
1 cup Nutella, divided
4 large eggs, divided
1 (18.25 ounce) box yellow cake mix (I used Duncan Hines)
1/2 cup buttermilk
Preheat oven to 350 degrees F. Lightly grease and flour 10-inch Bundt pan.
In small bowl, whisk together 2/3 cup Nutella and 1 egg. Blend until completely smooth and set aside. In large bowl, using electric mixer on low speed, stir together cake mix, remaining 3 eggs, buttermilk and remaining 1/3 cup Nutella until moistened. Scrape down sides and beater(s) and increase speed to medium, beating for another 2 minutes.
Remove 1 cup batter and mix with reserved Nutella-egg mixture. Pour leftover batter evenly into prepared cake pan. Spoon reserved Nutella batter over it and, using a knife, gently swirl batters together to create marble pattern. Smooth top of cake and bake for 40 to 50 minutes. (Cake is done when moist crumbs cling to inserted wooden pick.) DO NOT OPEN THE OVEN DOOR TO PEEK; THE CAKE WILL FALL!
Allow cake to cool in pan for 15 minutes. When ready to handle, carefully invert onto cooling rack and remove pan. While cake continues to cool, make Nutella Ganache. Pour Nutella Ganache over cake, allow ganache to cool completely, and serve.