Step 1

Preheat the oven to 350 degrees F (175 degrees C). Grease a deep 9x13-inch baking dish with cooking spray.

Step 2

Place yellow and red potatoes into a large pot and cover with salted water; bring to a boil. Cook until soft, about 10 minutes. Drain.

Step 3

Combine mozzarella cheese, Cheddar cheese, sour cream, and mayonnaise in a large bowl; mix well. Slowly add potatoes and toss; use a wooden spoon or flat spatula to avoid mashing the potatoes.

Step 4

Pour potatoes into the prepared pan. Dot butter over the top and cover with foil.

Step 5

Bake in the preheated oven, 25 to 30 minutes. Remove from oven and uncover. Preheat the oven's broiler. Return to the oven and broil until top is browned, about 5 minutes.