Pour in 1 cup water and add 1 1/2 tsp salt to the instant pot. Mix. Place a trivet into the instant pot bowl.
Place the peeled and cut up potatoes over the trivet. Cover with the lid and set the valve to ‘sealing’ position. Cook on manual high pressure for 12 minutes. When done, do a quick release. Once pressure has fully released, remove the lid then remove the trivet using a large fork or tongs. Don’t drain the water.
Work on the potatoes (which should be quite tender at this point) with a masher.
Pour over the heated or warmed-up light cream (half and half) or milk and continue to mix and mash. Adjust the amount of liquid to your desired consistency.
Add in the cubed butter and softened cream cheese and mix just until blended.
Use an electric hand mixer to achieve a smoother and lump-free consistency. Season to taste with more salt and a little pepper, if desired.
Serve right away if that's your plan. If you wish to make this ahead, transfer the mashed potatoes to a baking dish. A 9x13 pan should work best. Allow to cool completely then cover and chill. This can be made 1-2 days ahead.
Make sure to take the pan out of the fridge for a few hours so it reaches room temperature. I have used the slow cooker to warm the mashed potatoes up too, especially if the oven is occupied.
If not using the slow cooker, simply cover the dish and bake potatoes at a preheated 350 F oven for 50-60 minutes until heated through.
Before serving, cube the remaining butter (use between 1/4 to 1/2 cup butter, adjusted to your taste) and lay the slices on top of the mashed potatoes. The heat of mashed potatoes would melt the butter. Sprinkle with finely diced chives for garnish, if desired.