Slice the ends off of the squash. Then cut the squash in half lengthwise and cut each of those in half widthwise, then cut those in 2, until you have a total of 8 pieces.
Scoop out the seeds in the center cavity of the squash.
Place the trivet inside your Instant Pot and add 2 cups of water.
Add in the butternut squash on the trivet and close the lid.
Set the cook time for 15 minutes on high pressure.
Once cook time has elapsed, hit cancel and allow pressure to release naturally. When valve falls down and steam is gone, open the lid.
Remove the squash from the IP and gently remove the skin from the squash.
With a fork or a potato masher, mash the squash until the lumps are gone and texture smooths out. If you want it smoother you can also use an electric mixer, but I suggest leaving a bit of texture into your mashed butternut squash.
Continue by placing the squash into a large bowl. Add in the butter and stir until it is melted.
Add in the salt, pepper and sour cream, mix until combined and serve.
Optional: garnish with fresh rosemary, chives, thyme or basil