In a medium bowl, add the mayonnaise, ranch dressing powder and lemon juice and stir together until combined. Add the chicken and stir to coat with the ranch mixture.
Preheat the broiler to high. Place the bread slices on a rimmed baking sheet and toast until golden-brown, 3 to 4 minutes (watch the bread closely, as broiler intensities vary). Remove 4 slices from the baking sheet and set aside. Turn the remaining 4 slices of bread over so that the crisp, golden side faces down. Top each bread slice with 1 piece of cheese and return to under the broiler until the cheese is melted, 1 to 2 minutes (watch the cheese closely, as broiler intensities vary). Remove the baking sheet from the oven.
Divide the mashed avocado over the untoasted side of the 4 non-cheesy bread slices. Top each of the bread slices with the chicken salad and 2 lettuce leaves placed cupped side up. Add ¼ cup Cool Ranch Doritos® to each of the 4 uppermost lettuce cups. Top each bread slice with a cheesy bread slice, slice on a bias, and serve with more Cool Ranch Doritos® on the side.
For safe meat preparation, reference the USDA website.