Wash, peel and cut the potatoes into quarters (if using large potatoes, cut each potato into 8 pieces - the pieces should be about 2 inches square).
Add the potatoes to the Instant Pot and pour the water in too.
Add the salt and garlic cloves and stir everything together.
Add the lid, set the valve to sealing and set the Instant Pot to cook on High Pressure for 10 minutes.
After the cooking time is done, release the pressure manually (be careful to use a long-handled kitchen tool to move the valve to venting, and do it under a hood fan if you have one in your kitchen).
Open the lid when the pressure is released and drain any water from the Instant Pot.
Add the sour cream, milk (if using), and butter to the Instant Pot on top of the potatoes. If your dairy was cold from the fridge, add the lid and let the dairy warm up for 1-2 minutes before mashing.
Add the salt and pepper and mash the potatoes by hand or use a hand mixer and mix on low speed just until the potatoes are smooth. Be careful not to over-mix as this can create a glue-y or gloopy texture. Mix or mash just until no lumps remain.
Serve with a pat of butter on top of the hot potatoes and some fresh chopped parsley or chives, if desired (optional).