Prepare the sweet potatoes: Peel the sweet potatoes and cut them into roughly 1- to 2-inch chunks so they cook evenly and quickly in the Instant Pot.
Add water to the Instant Pot: Pour 1 cup of water into the Instant Pot insert and place the trivet inside. This prevents the potatoes from sitting directly in water while allowing steam to circulate.
Arrange the potatoes: Put the sweet potato chunks on the trivet or in a heatproof bowl on top of the trivet. Stacking is fine, but try to keep them in one layer if possible for even cooking.
Seal and cook: Close the lid and set the valve to sealing. Cook on high pressure for 8 minutes for medium-sized chunks (increase to 10 minutes for very large pieces).
Quick release: When the cooking time is up, perform a quick release by carefully moving the valve to venting. Once the pressure is fully released, open the lid.
Drain and mash: Transfer the cooked sweet potatoes to a large bowl and drain any excess moisture. Add butter and mash with a potato masher or ricer until mostly smooth.
Finish with dairy and seasoning: Stir in milk or cream a little at a time until you reach the desired consistency. Season with salt, pepper, cinnamon (if using), and maple syrup (if using). Taste and adjust seasonings.
Serve warm: Spoon the mashed sweet potatoes into a serving dish, dot with an extra pat of butter if you like, and sprinkle with chopped chives or parsley for color.