Step 1Heat the oven to 350 degrees. Line mini-muffin pans with paper cups, or grease and dust with unsweetened cocoa powder.
Step 2Put the chocolate and one-fourth cup of the oil in a glass bowl and melt in the microwave for 2 to 3 minutes at low heat, or place in the top of a double boiler over low heat, taking care not to scorch.
Step 3In a medium bowl, sift the flours, cocoa, baking powder, baking soda and salt.
Step 4In the bowl of an electric mixer, blend one-fourth cup oil and the sugar together till smooth. Add the eggs one at a time, beating well after each addition, then beat in the vanilla. Gradually add the flour mixture and buttermilk, beating until well blended, then mix in the chocolate and oil mixture. Add the zucchini and mix on low speed till just combined.
Step 5Fill each of the mini-muffin cups to three-quarters full and bake until a toothpick inserted in the center comes out clean, 10 to 12 minutes, or 18 to 20 minutes for standard muffins. Cool in the pans for about 10 minutes on a wire rack, then invert on the rack, remove the pan, and cool completely.
Step 6Dust with powdered sugar.