Slice the butternut squash vertically. Scoop out the seeds using an ice cream scoop.
Place water at the bottom of the Instant Pot, and place the trivet on top. Place both halves of the squash on the trivet (you may need to trim the top and bottom so it will fit). Secure the lid.
Select manual and cook on high pressure for 15 minutes. Once cooking is complete, use a quick release. Remove the lid once steam is completely released.
Use pot holders to carefully remove the trivet from the pot. Gently remove the skin of the squash (it should come off very easily), and place the squash inside a large bowl. Using a fork or pastry knife, mash the squash. Add butter or ghee and sea salt, continue to mash until smooth. To serve, sprinkle with additional salt if needed, and basil (if using).