Ground chipotle pepper (available at large supermarkets) gives these veggies a delicious, earthy smokiness, but ordinary chili powder works well, too.
Cut any large potatoes in half. In a covered medium saucepan, cook potatoes in a large amount of lightly salted boiling water for 10 minutes, adding the squash and mushrooms for the last minute of the cooking time. Drain and cool slightly.
Meanwhile, in a small bowl, combine butter, rosemary and chipotle chile pepper; set aside. On eight 10- to 12-inch metal skewers, alternately thread potatoes, squash and mushrooms, leaving a 1/4-inch space between pieces. Brush seasoned butter over vegetables. Sprinkle kabobs with salt.
Place kabobs on the rack of a gas or charcoal grill directly over medium heat. Cover and grill for 8 to 10 minutes or until vegetables are tender and brown, turning and brushing occasionally with the seasoned butter.