Place trivet in the bottom of your instant pot. Add one cup of water to the pot. Arrange potatoes on top of the trivet.
Place lid on the pot, slide into the locked position and turn the vent to the sealed position.
Set to PRESSURE COOK or MANUAL for 18 minutes.
Once time is up, natural release (don't do anything) for 5 minutes, then quick release (turn vent to the venting position and remove the lid once the float valve drops).
Slice the potatoes down the center to start cooling, then serve with butter and cinnamon-sugar or honey. OPTIONAL STEP: add marshmallows to the center of the open potatoes and transfer to oven to broil for 3-5 minutes until marshmallows are golden on top (watch closely so they don't burn).
If using sweet potatoes thicker than 2-2 1/2 inches, add two minutes onto the pressure cooking time.