Preheat the oven to 400 degrees F. Grease a 2-quart baking dish with cooking spray.
Peel the potatoes and cut them into 1-inch cubes. Place the potatoes and salt in a pressure cooker with water to cover and bring to a boil. Reduce the heat and simmer the potatoes until very tender, about 5 minutes. (If you do not have a pressure cooker, boil the potatoes in a pot of water with a pinch of salt until tender, about 20 minutes.)
While the potatoes boil, cook the bacon in a medium skillet over medium heat until browned and cooked through, 8 to 10 minutes. Drain the bacon on paper towels and, when cool enough to handle, crumble it into small pieces. Set aside.
Add the milk, mayonnaise, sour cream, butter, garlic salt, pepper and 1 cup of the cheese to a stand mixer with a paddle attachment. Whip the mixture until thoroughly combined. Drain the potatoes and add to the mixer. Mix again to combine and the potatoes are whipped smooth.
Spoon the mixture into the prepared baking dish. Sprinkle the crumbled bacon and remaining 1 cup cheese over the top and bake until the cheese is melted and the casserole is heated through, about 20 minutes.