Preheat oven to 375°F. Grease a loaf pan.
In a small pan over medium heat, melt 1 tbsp butter. Add chopped sage and cook for 1 to 2 minutes until fragrant and slightly crisp; remove from heat.
Whisk cornmeal, flour, sugar, baking powder, baking soda, and salt together in a bowl.
In a separate bowl, whisk squash purée, melted butter, sage, eggs, and buttermilk until blended.
Stir wet ingredients into dry until just combined. Fold in white cheddar if using.
Scrape into the loaf pan and bake for 30 to 35 minutes until golden brown on top and a toothpick comes out clean. Serve warm with butter.