Preheat oven to 425 degrees. Generously butter a rimmed baking sheet.
Halve squash crosswise. Scoop out seeds; discard. Slice a small piece off bottom of each squash half just enough to level.
Set squash halves, scooped sides down, on prepared sheet. Bake until golden, 20 to 25 minutes. Turn squash; prick insides all over with a fork. Divide 2 tablespoons butter and sugar among halves; season with salt and pepper. Continue to bake until flesh is easily pierced with the tip of a paring knife, 25 to 30 minutes. Serve warm.