To make the buttercup filling: Preheat oven to 350F.
Line a baking sheet with parchment paper.
Slice your buttercup squash in half carefully.
Scoop out all of the seeds and sprinkle the inside with a bit of salt. Place each half, cut side down on the baking sheet; rub some olive oil on the rind.
Bake for 30 minutes or up to an hour (or more), depending on the size of your squash. It is done when the flesh yields easily to an inserted fork.
Using a handheld mixer, blend the filling ingredients together until creamy and smooth. Some squashes will be creamy and soft without adding much almond milk. Add only as much almond milk as you need to achieve a nice creamy, scoopable texture.
To make the cake: Preheat oven to 350F.
Spray your pan and set aside. This recipe will work in a regular muffin pan as well.
Sift dry ingredients together. In a separate bowl mix your wet ingredients (except for the apple cider vinegar).
Slowly pour the wet ingredients over the dry ingredients, whisking continuously.
Whisk in the apple cider vinegar.
Pour your batter into your prepared pan, filling only ½ way.
Using a cookie dough scoop or a spoon, place your buttercup filling in the center of each cake. Roughly 2 tablespoons of filling per cake.
Pour your remaining batter over the top of the cakes. If you are using a regular muffin pan there will be a bit of extra batter that you can use to make a mini loaf.
Bake for 35 - 40 minutes (or 25 - 30 minutes for a muffin pan) or until a toothpick inserted comes out clean.