Light a grill or heat a medium cast-iron skillet. Grill or sear the tomatillos and garlic over moderate heat until the tomatillos have blackened and the garlic has softened, 7 to 8 minutes for the tomatillos and 12 minutes for the garlic. Peel the garlic and transfer to a food processor. Add the tomatillos, chipotle and sugar and puree until smooth. Season with salt and serve.
The salsa can be refrigerated for 1 day. Let return to room temperature before serving.