Preheat the oven to 375 degrees F. Grease 12 standard-sized muffin tin cups or coat with non-stick spray. (Or, use paper muffin cup liners, if desired.)
In a food processor combine the flours, 1 cup sugar, cocoa, baking powder, salt, and 1/2 cup chocolate morsels. Process until the morsels are finely ground. With the motor running, slowly pour in the oil and process until evenly incorporated; stop and scrape up the mixture on the bottom several times if necessary. Turn out the mixture into a medium bowl. Process the zucchinis in the processor until finely chopped. Then measure; you should have 1 1/2 lightly packed cups. If necessary chop enough more zucchini or discard extra so you have 1 1/2 cups. Add the chopped zucchini to the dry ingredients. Add the yogurt and eggs to the processor; process to blend. Stir the egg mixture, remaining chocolate morsels, and nuts into the flour, mixing gently just until evenly incorporated; don't over-mix or the batter may toughen.
Using a 1/2-cup measure or large spoon, immediately divide the batter among 12 muffin cups; they should be full. Sprinkle the muffin tops with the reserved 1 1/2 tablespoons sugar, dividing it equally among them.
Bake in the middle third of the oven for 20 to 24 minutes or until the muffins are springy to the touch, and a toothpick inserted into the thickest part of a center muffin comes out clean. Cool on wire rack 3 or 4 minutes; if no paper liners were used, gently run a knife around cups, then remove muffins. Let stand until cooled. They are best when fresh, but can be kept airtight for a day or so. (They may also be frozen; thaw completely before serving.)
Makes 12 generous standard-sized muffins.