Preheat the oven to 400°F. Optional: Line a baking sheet with parchment paper.
Halve acorn squash top to bottom. Scoop out seeds and stringy flesh.
Divide butter between squash halves, rubbing into the cut flesh with the back of a spoon. Sprinkle on the brown sugar and use the spoon to press it into the butter.
Place the squash cut-side up on the baking sheet. Roast in the oven for 30-40 minutes, until soft but not falling apart. Use tongs to carefully transfer to a serving dish or plate.
Notes
If using unsalted butter, add a small pinch of salt to each squash half.
Total roasting time will depend on the size of your squash.