Canned "pumpkin" is actually made with different species of winter squash than what we typically think of as pumpkins. Which means you can pick any sweet, dense squash you choose and use it anywhere pumpkin purée or canned pumpkin is called for! There are plenty of different options if you don't want to use butternut, like acorn or kabocha squash, plus tons of uses for the leftover seeds, guts, and peel.
I bake them at a slightly lower temperature than normal (375°F) to avoid any browning. Although the caramelization adds flavor, it can also add dark flecks to the purée. You should plan on getting roughly 1 cup of purée per pound of squash; I oftentimes get more than that, but better safe than sorry. Luckily, any leftover squash can be stored in the fridge in an airtight container for about a week, or frozen for much longer.
One other thing to make a note of: If you start with a dense squash, you should have a dense purée, but if it turns out watery there's an easy fix. I picked up this trick from longtime Food52er HalfPint on the Hotline. She writes: "What you can do before you use the purée in baked goods is squeeze out the extra moisture. America's Test Kitchen spreads pumpkin purée onto paper towels and then squeezes or pats out the moisture. It may sound difficult and messy but it's quite easy. The purée will almost peel off of the paper towel. You can also use a clean/pristine kitchen towel and do the same."
Cut butternut squash in half lengthwise and scoop out the guts and seeds; I like to use a grapefruit spoon (don't toss what you scoop out!). Rub each cut side with 1/2 teaspoon olive oil and place cut side down on a baking sheet either directly or on parchment paper for easier cleanup.
Bake until the squash is very tender, about 30 to 45 minutes (check by piercing with a sharp knife). Let stand until the squash is cool enough to handle.
Separate the squash flesh from the peel. Sometimes the skin easily peels right off, but if it doesn't, I use the grapefruit spoon again.
Purée the squash flesh. Yes, you could just mash it up with a fork or potato masher, but I like to get it super smooth. Use a food processor, or, my preference, an immersion hand blender (put the squash flesh in a tall, deep container to reduce splatters).