Crush chipotle in mortar and pestle, or grind in a spice grinder. Add chipotle, chili powder, cumin, cayenne, garlic powder, sugar and salt to yogurt and stir really well. Taste for seasoning; each ear only gets a thin coat, so if you want it hot, add more cayenne.
Strip corn of husks and silk. Arrange on broiler tray in a single layer. Brush all sides with butter and broil 5 minutes. Turn over and broil another 5 minutes. Brush all sides with butter again. Continue cooking and turning often until corn is golden and a few kernels are charred.
Transfer corn to a platter. Smear each ear with about 2 tbsp of yogurt mixture, spreading evenly. Sprinkle all over with parmesan and squeeze lime wedges over. Eat hot.