Meanwhile, in a large saucepan, combine the water, milk and table salt and bring to a boil. Gradually whisk in the cornmeal and bring to a simmer. Reduce the heat to low and cook, stirring every few minutes, until the cornmeal is smooth, about 30 minutes. Stir in the butter, Parmesan and mashed butternut squash. Season the polenta with kosher salt and pepper and serve hot.