Preheat the oven to 400°F [204°C]Carefully cut the squash in half, from stem to blossom end. Use a large metal spoon to clean out the seeds and fibers out of the squash half cavities.
Soften the butter in a microwave until it is the consistency of thin icing.Use a pastry brush to paint the inside of the squash halves with soften butter. Reserve the remaining soften butter.
Place the squashes on a heavy baking sheet, glass baking dish, or open cast iron skillet.Sprinkle the buttered squash halves with kosher salt and ground black pepper.
Divide the remaining softened butter into the two squash cavities.
In a small bowl, combine brown sugar, cinnamon, and nutmeg. Sprinkle half of the mixture into each squash half.
Place the baking tray or skillet in the preheated oven. Bake for 50-60 minutes, until the squash is fork-tender and the flesh has just begun to caramelize.