Adjust the oven rack to the lower third position and preheat the oven to 350°F. Grease an 9 x 5 inch loaf pan with butter and dust with flour.
In a medium bowl, combine the flour, sugar, pumpkin pie spice, baking soda and salt and whisk until combined.
In a large bowl, mix together the butternut squash, vegetable oil, applesauce, eggs and vanilla extract until combined. Add the flour mixture and mix until just combined. If adding using chocolate chips and walnuts, gently fold them in at this time.
Add the batter into the pan, making sure it is even. Bake for 65-75 minutes, covering with aluminum foil halfway through if bread is browning too quickly. At 65 minutes, start checking to see if bread is done every 5 minutes by inserting a toothpick or cake tester in the center where it comes out clean with just a few moist crumbs.
Let the loaf cool in the pan for 10 minutes before removing and placing on wire rack.
Use baking soda that is less than six months old since it has been opened. If you use older baking soda, it won’t be powerful enough to help leaven the bread. You can test it by stirring half a teaspoon of baking soda into a cup of hot water. It will immediately start to fizz and release carbon dioxide gas if it’s still fresh enough to use.
If using nuts, don't skip out on roasting them. By roasting the nuts, they will stay crispy in the batter and bring out the flavor.
If the bread is browning too quickly, place some aluminum foil loosely on top.
If you want to freeze the bread, wrap the loaf in 2-3 layers of plastic wrap or aluminum foil and then place in a freezer-friendly storage bag. Freeze up to 3 months and then thaw overnight in the fridge before serving.