Pierce butternut squash skin with a small knife and place cut-side down in a baking dish; pour in enough water for a depth of 1/2 inch. Cover dish with aluminum foil.
Bake in the preheated oven until squash is tender, about 1 hour.
Scoop squash into a bowl; add 3/4 cup sour cream, butter, brown sugar, cinnamon, salt, and pepper. Mash mixture using a potato masher or fork until smooth. Garnish with parsley and 1 tablespoon sour cream.