Preheat oven to 350°F.
In a large mixing bowl, combine chocolate cake mix, brownie mix, eggs, water, oil, and mayonnaise. Sitr or mix with a hand beater until well combined. Fold in chocolate chips and stir to combine.
Lightly spray a 12 cup bundt pan with non-stick cooking spray. Spread batter evenly into bundt pan.
Bake for 45 to 50 minutes or until a toothpick inserted into the center of the cake comes out clean (or with just a few crumbs).
Cool cake for 10 to 15 minutes; remove from bundt pan to cool the rest of the way on a cooling rack.
When cake is cooled, prepare ganache.
Heat heavy cream in a small saucepan or in a microwave safe container until steaming. Pour cream over chocolate chips in a small bowl. Cover with a plate for 2 to 3 minutes to contain the heat and allow the chocolate chips to soften.
Whisk until the mixture forms a smooth, glossy ganache.
Pour ganache over cake and serve.