Preheat oven to 425°F (220ºC). Spray a baking dish with non-stick cooking spray.
Halve the squash lengthwise. Scoop out seeds; discard. Set squash halves, scooped sides down, on prepared sheet. Bake until golden, 20 to 25 minutes (or microwave for 6 to 8 minutes cut-side down in 1/2inch of water).
Turn squash; prick insides with a fork. Spread butter and sprinkle brown sugar and cinnamon among halves and season with coarse salt. Continue to bake until flesh is easily pierced with the tip of a paring knife, about 20 to 25 minutes.
Serve directly after baking.