Preheat oven to 350 degrees. Line 3, 8" round baking pans with parchment paper. Butter and flour sides of pans (I use Baking PAM for this).
In a large bowl or base of stand mixer, beat butter, sugar and brown sugar on high speed until light and fluffy, about 3 minutes.
On medium speed, beat in eggs, one at a time, until well incorporated. Scraped down sides as needed. Then, beat in vanilla extract.
In a medium sized bowl, stir together flour, cornstarch, baking powder, baking soda, salt and cinnamon.
Alternate beating flour mixture and buttermilk into butter/sugar mixture, in about 3 to 4 additions. Start and end with flour mixture.
Divide batter evenly among prepared pans and cook until toothpick in center of pan comes out with just a few crumbs. About 27-30 minutes. NOTE: Tops are a dark golden brown when done.
Let cool at least 5 minutes before removing from pans.