Prepare pans with butter and flour. Place a circle of buttered parchment in the rounds.
In a large bowl or stand mixer bowl, cream the butter and sugar, then add the oil. Add the eggs and yolks one at a time, being sure to completely incorporate each egg before adding the next.
In a second bowl, sift together flour, baking powder, salt and cinnamon. Add this slowly to the butter, starting with about a third of the flour bowl, followed by half the buttermilk, another third of the flour, the last of the buttermilk and finally the rest of the flour. After your last addition of flour, do NOT overmix, just until the flour disappears.
Spoon batter evenly into prepared round pans or completely into the 9 x 13 pan. Bake off about 30-35 minutes or until a toothpick comes out clean and the middle springs back to the touch in the center. Do not overbake.
Cool on a rack, in pan(s), then remove and place on wire rack to cool completely before frosting.