Place butter and sugar into a large mixing bowl, cream until well combined. Add sour cream, vanilla and egg, stirring to combine. Stir in shredded zucchini then stir in flour, cocoa powder, baking soda, baking powder, salt and chocolate chunks. Once combined, fill muffin liners 3/4 full and bake for 20 minutes, until baked through. Remove and let cool for 10 minutes.