Preheat your oven to 350°F (175°C).
Grease and flour a 9x5 inch loaf pan well.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and pumpkin spice blend. Ensure everything is evenly distributed.
In a separate medium bowl, whisk together the eggs, sugar, vanilla extract, vegetable oil, buttermilk, and butternut squash puree until nicely combined.
Pour the wet ingredients mixture into the bowl with the dry ingredients. Stir gently with a spatula or spoon until you no longer see any dry flour streaks. Do not overmix; a few lumps are fine.
Pour the batter into your prepared loaf pan and spread it out evenly.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Once done, take the pan out of the oven and let the bread cool in the pan for about 10 minutes.
Carefully turn the loaf out onto a wire rack to cool completely.
(Optional Glaze) While the bread cools (or once it's cool), whisk together the icing sugar and your chosen liquid in a small bowl until smooth and drizzle-able. Add liquid a tablespoon at a time until the desired consistency is reached.
(Optional Topping) Once the bread is completely cool, drizzle the glaze (if using) over the top and sprinkle with a little extra pumpkin spice blend or cinnamon (if using).
Slice and enjoy!