Arrange pepper halves skin side up or whole peppers on a broiler pan coated with nonstick cooking spray. Broil on the closest rack position to the heat without the peppers touching the heat source. Broil until skins are blistered and blackened, about 10 minutes. For whole peppers, with tongs, rotate peppers a quarter turn. Broil and rotate until all sides are blistered and blackened.
Immediately place peppers in a bowl; cover with plastic wrap and let stand for 15-20 minutes.
Peel off and discard charred skin. Remove stems and seeds from whole peppers.