In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Stir in vanilla. Combine the flour, baking cocoa, baking powder, baking soda and salt; add to the creamed mixture alternately with yogurt, beating well after each addition. Fold in zucchini and carrots.
Fill paper-lined muffin cups two-thirds full. Bake at 350° for 18-22 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely. Frost cupcakes.