For the cake: Preheat the oven to 350 degrees F. Grease a 10-cup Bundt pan with baking spray.
Mix together the cake mix, milk, butter, eggs, chocolate chips and vanilla in a large bowl until completely combined. Pour into the prepared Bundt pan and bake until a toothpick inserted in the center comes out clean, about 40 minutes.
Let the cake cool for about 15 minutes before turning it out onto a cooling rack. Allow to cool completely, an additional 20 to 25 minutes.
For the simple chocolate ganache topping: Meanwhile, heat the cream in a small pan until just bubbling. Add the chocolate chips to a small bowl and pour the hot cream over the top. Let sit for 1 minute, then mix until smooth.
Transfer the cake to a cake stand or serving platter and pour the ganache over the top, allowing it to drip down the ridges. Sprinkle with rainbow sprinkles and serve.