1. Preheat the oven to 350°F and grease a 10-cup Bundt pan very well.

2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

3. In a large bowl, beat the butter and sugar until light and fluffy.

4. Add the eggs one at a time, mixing well after each addition.

5. Mix in the vanilla extract, orange zest, and orange juice.

6. Add the dry ingredients in batches, alternating with the sour cream and milk, and mix just until combined.

7. Transfer about one-third of the batter to a separate bowl and stir in the cocoa powder until smooth.

8. Spoon part of the orange batter into the prepared Bundt pan, add the chocolate batter in sections, then top with the remaining orange batter.

9. Swirl the batters gently with a knife to create a marbled effect.

10. Bake for 45 to 55 minutes, or until a toothpick inserted into the cake comes out with a few moist crumbs.

11. Cool the cake in the pan for 15 minutes, then turn it out onto a wire rack to cool completely.

12. Heat the heavy cream until warm, pour it over the chocolate chips, and stir until smooth.

13. Spoon the chocolate glaze over the cooled cake and let it set before slicing.