Place an oven rack just below center and preheat to 325 degrees. Grease a Bundt cake pan thoroughly with cooking spray and set aside. In a large bowl, or the bowl of a stand mixer, cream butter and sugar together for about 5 minutes until very light and fluffy.
Add vanilla and eggs, one at a time, mixing well after each addition. Stir in sour cream and set aside.
Sift flour, baking powder, baking soda and salt together in a separate bowl. Add half of flour mixture to wet mixture and fold gently to mix. Pour in milk and stir until combined, and then fold in the remaining flour mixture until just combined. Divide cake batter evenly between the 2 bowls. Stir ¼ cup cocoa powder into one half of the batter.
Dollop large scoops of cake batter into the prepared Bundt pan, alternating between the chocolate and vanilla. Once all batter is in the pan, tap pan on the countertop to force any trapped air bubbles out and to help settle batter into the pan.
Use a skewer or butter knife to drag and swirl batter together to create a marbled effect. Bake 50–60 minutes, until a cake tester comes out with a few moist crumbs attached. Remove from oven and allow the cake to cool in the pan for 5 minutes before turning it out onto a cooling rack to cool completely.
When cake is completely cool, place chocolate chips, butter and honey into a microwave-safe bowl. Heat the chocolate in 20-second intervals, stirring after each until chocolate is melted. Add vanilla and stir until the glaze is smooth and glossy. If the glaze is a little thick, heat it just a few more seconds until it is a thick but pourable consistency.
Pour glaze evenly over the top of the Bundt cake and allow glaze to set about 10 minutes before slicing. Enjoy!
One Last Thing
You can also make this marble cake in a regular 9" x 13" pan! Just bake at 325 degrees for 40–45 minutes.