Place fresh tomatillos in a medium saucepan and add water to cover (about 3 cups). Bring to a boil and cook uncovered on high heat until tomatillos have changed color, about 5 minutes. Drain, reserving l/2 cup of the cooking liquid. If using canned tomatillos, omit this step.
In small heavy skillet, heat the oil until almost sizzling and fry the tortilla until crisp and golden. Drain it on paper towels and when cool enough to handle, break into pieces.
Place the cooked tomatillos with the reserved liquid in container of blender. (If using canned tomatillos, no extra liquid is necessary.) Add the chopped onion, garlic, chiles, cilantro, broken tortilla pieces, and sugar. Process for about l minute, or until smoothly pureed.
In a medium saucepan, heat the butter over high heat until hot and bubbling. Add the tomatillo mixture and heavy cream. Season with salt to taste. Bring to a boil over high heat; reduce heat to low and simmer for 5 minutes or until somewhat thickened.
Can be kept up to a week tightly covered in refrigerator. It may break down and look curdled but will smooth out when reheated.