Preheat oven to 325°F. Grease and coat pan in cocoa powder, or spray and line cake pans with parchment.
In a bowl, whisk together the flour, cocoa powder, salt and baking powder. In a separate bowl, whisk together heavy cream and sour cream. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, sugar and light brown sugar until slightly lightened.
Beat in the eggs one at a time, scraping down after each addition. Add the vanilla extract with the last egg.
Add all the chocolate and beat well to incorporate.
Alternately add the dry ingredients and sour cream mixture until just barely combined. Remove bowl from stand mixer and fold in the zucchini just until distributed.
Spoon into prepared pan(s) and bake in preheated oven. The 10 cup bundt pan that I used, baked about 45 minutes. A cake tester will come out with a few clinging crumbs but will not look wet.
Allow to cool to room temperature before unmolding.
Unmold on your favorite cake stand or plate. If your cake needs a bit of help coming out of the pan, use a small offset spatula to gently free the sides and edges. Patience is key here.
Dust with cocoa powder or glaze with your favorite icing or chocolate sauce!
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Notes
This recipe is for a 10 cup bundt pan. You can double it for a large bundt pan.