Do you cook butternut squash with the skin still on or do you have to peel it first?
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You can do either. You can cut it in half, scrape out the seeds, place on a tray and roast it in the oven.
Or if you want cubes, you woulc cut it into sections, set a section upright and carve down the sides with a sharp knife to remove the peel.
When I want just the mashed up squash puree, I've microwaved the whole unpeeled squash with great results: Pierce skin all over with knife/fork. Place on microwave-safe dish. Nuke for about 20 - 30 minutes, depending on size, turning occasionally, until very soft. Slice in half and if squash is still firm in the middle, nuke for a few more minutes. Then scrape out pulp and use as needed.
Similar to mrslarkin, I put the squash in the microwave, but I cut it in half first. It still takes about 20-30 minutes, but I'm always afraid that I am going to make the squash explode if whole!