The key to success with this favorite Mexican recipe, is to fry the cheese-stuffed peppers until the outside is crisp and golden and the cheese inside is melted and gooey.
Place chiles directly onto grate of gas burner over medium-high heat. Blacken chiles, turning frequently, about 5 minutes total. (Or blacken under a broiler.)
Transfer chiles to a bowl; cover with plastic wrap. Let rest 5 minutes to soften. Uncover and cool to room temp. Peel skin from chiles, using a paper towel to help strips of skin separate from chile.
With small knife, cut a slit down long side of chiles. Remove seeds and membranes, trying not to tear chiles.
Cut cheese into 16 thin slices. Tuck 2 slices into each chile and secure opening with toothpick.
Beat whites, salt and cream of tartar to stiff peaks. Fold in yolks.
Heat oil in a large skillet over medium-high heat. Dip one chile into egg mixture, then transfer to hot oil. Repeat with two to three more. Cook about 4 minutes, turning on all sides. Repeat with remaining chiles, keeping cooked chiles warm in 200°F oven.
Heat enchilada sauce in a small saucepan or in microwave. Transfer hot chiles to a serving platter and top with sauce and sour cream, if desired.
To save time, use canned whole green chiles instead of prepping your own.