Step 1: Preheat your oven to 400°F (200°C) and roast a halved butternut squash for 45 minutes until soft. Let it cool, then scoop and mash into a puree.
Step 2: Lower the oven temperature to 350°F (175°C) and grease a 9-inch round cake pan or loaf pan.
Step 3: In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
Step 4: In a separate bowl, cream the softened butter and dark brown sugar until light and fluffy.
Step 5: Beat in the eggs one at a time, followed by the vegetable oil, vanilla extract, and the roasted butternut squash puree.
Step 6: Gradually fold the dry ingredients into the wet mixture until just combined. Do not overmix.
Step 7: Pour the batter into the prepared pan and bake for 45-55 minutes, or until a toothpick inserted into the center comes out clean.
Step 8: Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.