In a stand mixer bowl add oil and sugars. Mix on medium speed. Add in eggs and vanilla. Blend again. Add in mashed sweet potatoes and blend well. Add in dry ingredients. As the batter is mixing, add in buttermilk. Mix until all ingredients are incorporated. Pre-heat oven to 350. Butter and flour three 6 inch pans. Divide batter between the 3 pans and bake for 35-40 minutes until the centers are risen and fully baked. Cool for 10 minutes, then invert onto a wire cooling rack. Cool completely. Use a knife to cut off the domes and level the cake layers.