Put butter and brown sugar in separate resealable bags.
Build a good bed of coals.
Place butternut squash on a medium-hot bed of coals.
Cook, turning occasionally to avoid hot-spot and to char on all sides. About an hour.
Ten to fifteen minutes before the squash is done, heat the butter until melted (but, not boiling) in skillet.
Add brown sugar to melted butter. Cook until melted. Don't boil.
Test to see if squash is done by inserting metal probe or small knife. If the squash appears soft all the way through, then remove from coals.
Slice squash down the middle longways and remove seeds with a spoon.
Add salt and pepper to taste and drizzle with the butter/brown sugar mixture. Serve.