Preheat the oven's broiler and set the oven rack at about 6 inches from the heat source.
Cook under the preheated broiler until the skin of the peppers has blackened and blistered, about 8 minutes per side. When the peppers are about blackened, place them a plastic bag, seal, and allow the peppers to steam as they cool, about 30 minutes. Once cool, remove the skins and discard. Cut a slit lengthwise into the side of each pepper, and remove seeds and veins.
Divide the cheese into 8 portions, and squeeze each portion lightly in your hands to make a cone shape. Insert a cone into the slit in each pepper, and pin the openings closed with toothpicks.
Dredge each pepper in flour, dust off the excess, and place on a small baking sheet lined with parchment paper or waxed paper. Place the peppers in the freezer for at least 1 hour.
To make sauce, blend the diced tomatoes in a blender until pureed, and set aside. Heat 1 tablespoon of butter in a large pan over medium heat, and cook and stir the onion until translucent, about 5 minutes. Add the garlic, cook and stir for about 30 seconds, and pour in the pureed tomatoes, chicken broth, taco seasoning, & hot pepper sauce. Reduce to medium-low, and simmer the sauce about 45 minutes, stirring often.
To make batter and finish the chiles rellenos, mix 1/3 cup flour with salt in a bowl. In a separate bowl, beat the egg whites with an electric mixer until the whites form stiff peaks. Lightly mix the egg yolk and flour-salt mixture into the egg whites to make a fluffy batter.
Remove peppers from freezer and coat them in flour again; shake off excess flour, and dip each pepper into the egg batter, using a spoon to gently but completely coat each pepper with batter.
Heat frying oil in a large skillet over medium-high heat until the oil shimmers. Gently lay the coated peppers into the hot oil, and fry on both sides until the batter is golden brown and the cheese filling is hot, about 5 minutes per side.
To serve, ladle a generous spoonful of sauce onto plate, and place pepper on sauce.