Preheat oven to 350ºF (177ºC). Generously spray a 12-cup bundt pan with nonstick cooking spray.
In a large mixing bowl, add cake mix (1 box), sugar (1/3 cup), sour cream (8 ounces), and oil (2/3 cup), and mix on low speed for 1 minute.
Add the eggs (3), one at a time, blending well after each addition.
Increase mixer speed to medium and beat for 4 minutes.
Add vanilla (1 teaspoon) and chocolate chips (2 cups), and mix on low speed until the chips are evenly distributed.
Pour the batter into prepared pan and bake in preheated oven 60-65 minutes or until a tester inserted into the middle of cake comes out clean.
Allow cake to cool in pan then invert onto a cake plate.
When cake is cooled, make ganache.
In a glass bowl, combine the chocolate (6 ounces) and butter (2 tablespoons).
In a small saucepan over medium-high heat, bring the cream to a boil. While the cream is heating, microwave the chocolate and butter for 30 seconds at 100% power. Immediately pour the heavy cream (1/2 cup) over the chocolate and butter. Whisk until the mixture melts and becomes smooth.
Pour the ganache over the top of the cake. Allow the ganache to set for about 20 minutes, garnish with chocolate chips (1/2 cup), then cut the cake and serve.