You can use fresh butternut squash that's roasted and then pureed smooth in a food processor, or canned butternut squash. If you use canned, just be sure to drain off any liquid that settles at the top. We love the flavor of fresh squash (and the convenience of using a piece of produce we have from the garden) but it's super delicious with canned squash too so no worries if you don't have access to a whole squash.