Preheat oven to 400 degrees F (200 degrees C). Place acorn squash, cut-side down, on a baking sheet.
Bake in the preheated oven for 30 minutes. Flip squash and sprinkle cinnamon into the center and dot with butter. Continue baking until flesh can be easily pierced with a fork, about 30 minutes more.
Scoop flesh into a bowl and mix in brown sugar until evenly combined.