Poach the chicken breasts in gently boiling water for about 25 minutes, then shred them into bite-sized pieces once cool enough to handle. The meat should pull apart easily with two forks and look opaque all the way through.

Preheat the oven to 350°F and coat a 9x9-inch baking pan with non-stick spray. A properly greased pan keeps the finished dip from sticking to the edges once it bakes.

In a saucepot over medium-low heat, whisk the cream cheese, ranch dressing, hot sauce, black pepper, and garlic powder together until fully smooth. The sauce should turn glossy and uniform with no visible lumps of cream cheese remaining.

Stir the shredded chicken, chopped green onion, and 1 cup each of mozzarella and cheddar into the warm sauce. The mixture should thicken slightly and offer gentle resistance when you stir, a sign the cheese is starting to melt in.

Transfer the mixture into the prepared pan, scatter the remaining cheese over the top, and bake for 20 to 30 minutes until the edges turn bubbly. Finish with a 2 to 3 minute broil to brown the top into a golden, slightly crisp crust, watching closely so it doesn't scorch.

Serve the dip hot straight from the pan. Pair it with tortilla chips, celery sticks, and carrots for scooping.