Pierce the squash all over with a fork. Place it on a microwave-safe plate. Microwave until fork-tender, 10-15 minutes, turning halfway through cooking.
Transfer the cooked squash to a cutting board. Be careful - the plate and the squash will be hot. Allow it to rest until cool enough to handle, about 10 minutes.
Using a sharp chef's knife, carefully cut the squash lengthwise. Use a spoon to remove the seeds and pulp.
Rake a fork back and forth over the squash halves to remove the flesh in strands. Place the strands in a bowl and set them aside. You should have about 4 cups of strands.
Heat the butter in a large saucepan over medium-high heat. When the butter melts and starts to foam, add the spaghetti squash strands, kosher salt, black pepper, and garlic powder. Cook, stirring, until the noodles are well-coated, 1-2 minutes.
Turn the heat off and stir in the parmesan cheese. Serve immediately.
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Notes
You definitely don't want to overcook the squash. While the noodles won't be al-dente like wheat noodles, you don't want them to be overly mushy. So check on the squash while you cook it (whether using the microwave or the oven) to ensure it doesn't get overcooked. And when you saute the noodles with butter and seasonings, do so briefly.
You can keep the leftovers in the fridge, in an airtight container, for 3-4 days. Reheat them gently in the microwave at 50% power. You can also freeze the leftovers for up to three months.