First, make the cakes. Heat the oven to 350F, and grease & flour 4 6-inch cake pans {or 3 9-inch pans}.

In the bowl of a stand mixer, beat together the butter and sugars until light and fluffy. Mix in the oil. Add in the eggs, one at a time, beating until each addition, and finally stir in the vanilla.

In a medium bowl, mix together the mashed sweet potato and milk until smooth.

In a separate medium bowl, whisk together the flour, baking powder, baking soda, salt, and pumpkin spice.

Add half of the dry ingredients into the butter mixture, and mix until combined. Add in the mashed sweet potato mixture and mix again, then add in the remaining dry ingredients.

Divide the cake batter evenly among the prepared cake pans, and bake for 40-45 minutes for 6-inch pans, or 30-35 minutes for 9-inch pans.

Let cakes cool slightly, then remove from pans and let cool completely before frosting.

When ready to make the frosting, beat the softened butter and marshmallow creme in the bowl of a stand mixer until light and fluffy. Mix in the vanilla and salt, then beat in the powdered sugar ½ cup at a time. Finally, add in milk until the frosting is as stiff or soft as needed!

Frost cake as desired, and serve!