Pierce the spaghetti squash all over with a fork. Place on a microwave-safe plate. Microwave until fork-tender, 10-15 minutes, turning halfway through cooking.
Transfer the cooked spaghetti squash to a cutting board. Allow to rest until cool enough to handle, about 10 minutes.
Using a sharp knife, cut the spaghetti squash lengthwise. Use a spoon to remove the seeds and pulp.
Rake a fork back and forth over the spaghetti squash halves, to remove its flesh in strands. Place the spaghetti squash strands in a bowl and set aside.
Place the butter in a large saucepan. Heat over medium-high heat. When butter melts and starts to foam, add the spaghetti squash strands, Kosher salt, black pepper, and garlic powder. Cook, stirring, until the spaghetti squash strands are well-coated, 1-2 minutes.
Turn the heat off and stir in the Parmesan cheese. Serve immediately.