Add 1 cup of water to the Instant Pot insert and place a trivet or steamer basket inside.
Place the peeled and cut sweet potatoes on the trivet in a single layer as much as fits.
Close the lid and set the valve to sealing. Pressure cook on high for 8 minutes for 1- to 2-inch pieces. If pieces are larger, add 2 minutes.
When cooking finishes, quick release the pressure carefully by turning the valve to venting. Check doneness with a fork; potatoes should be fork-tender.
Remove the sweet potatoes to a large bowl, discarding the cooking water from the pot. Return the Instant Pot insert to keep it warm if needed.
Add butter, milk or half-and-half, ground cinnamon, salt, and pepper to the warm sweet potatoes.
Mash using a potato masher or hand mixer until creamy. For the silkiest texture, use an immersion blender for a few seconds, but avoid overworking it.
Taste and add maple syrup or honey if you like it sweeter. Adjust seasoning with more salt, pepper, or cinnamon.
Transfer to a serving dish and garnish with chopped parsley or chives if desired. Serve warm.