Lightly coat a small mixing bowl with cooking spray to serve as a mold for the polenta. Bring a small pot with 10 oz. water, butter, and 1 tsp. salt to a boil. Add polenta in a steady stream, whisking constantly until incorporated. Reduce to simmer and cook, stirring occasionally, until creamy, about 3-4 minutes. Stir in Parmesan, transfer to prepared bowl, and set aside to firm up, about 10-15 minutes.