Preheat oven to 325°F and spray a Bundt pan with nonstick spray. In a large bowl, beat the butter, oil and sugar on high speed with an electric mixer for 2-3 minutes, until thick and pale yellow. Beat in the eggs and vanilla.
In a smaller bowl, stir together the flour, cocoa, baking powder, baking soda and salt. Add a third to the butter mixture and beat just until blended; add half the sour cream and do the same. Repeat with another third of the flour mixture, the rest of the sour cream, and the rest of the flour. Stir in the zucchini and the nuts and/or chocolate chips, if you're using them, and pour the thick batter into the pan, smoothing the top.
Bake for an hour, until the top is cracked and springy to the touch. Invert onto a plate or cake stand while still warm.