Preheat the oven to 350 degrees F. and lightly grease a 9-inch round baking pan.
In a large bowl, combine the plant-based milk and apple cider vinegar. Whisk until it thickens and becomes buttermilk.
Add the oil, brown sugar, and vanilla extract. Mix well until the sugar is almost fully dissolved. Add the butternut squash puree and mix until all of the ingredients are fully incorporated.
In a medium bowl, combine the flour, baking powder, baking soda, salt, and pumpkin spice.
Add the dry ingredients to the bowl with the wet ingredients. Mix well, but do not overmix.
Pour the batter into the prepared pan and bake for 25 to 30 minutes or until a toothpick inserted in the middle comes out mostly clean.
Remove it from the oven and place it on a cooling rack to cool completely before removing from the pan.
Notes
I use this butternut squash puree, but if you’d like to make your own, this is how to do it: Place 2 cups of cubed butternut squash in a medium pot and cover with water. Bring to a boil and cook for 20 minutes or until soft. The smaller the cubes, the faster it will cook. Drain and allow it to cool before pureeing.
Measure the flour correctly: To avoid ending up with dense bakes, first, fluff it in its bag, spoon it into the measuring cup, then level the top with the back of a knife.
Avoid over-mixing: Over-mixing batter overworks the gluten in the flour and leads to dense, rubbery cakes. Mix until ‘just’ combined – a few small lumps are fine.