Take the washed, peeled and diced (into 1 inch cubes) and place them in the Instant Pot along with the water.
Lock the lid into place, make sure the knob is set to sealing and then pressure cook on Manual HIGH for 2 minutes and then quick release the pressure.
Drain any liquid that is left.
Add the butter to the potatoes and roughly mash by hand until the butter has been incorporated.
Add in the milk and sour cream and finish mashing the potatoes. Do not over mash!
Season with salt and pepper. I didn't write a specific measurement because this is something that varies so much person to person. I suggest seasoning, tasting, seasoning, tasting etc until it is seasoned to your liking.
Stir in 1/2 cup of the shredded cheese and then transfer to an oven safe dish.
Top the potatoes with the remaining cheese. Broil in the oven until the cheese has melted.