Preheat the oven to 350 degrees and butter and flour the bottoms and sides of a 9-inch springform pan.
In a large bowl, sift together the flour, cornstarch, baking powder, baking soda, and salt.
In the bowl of an electric mixer, beat together the butter, oil, sugar, and banana on medium speed until creamy, 1-2 minutes. Beat in egg yolks and vanilla until well combined. Reduce mixer to low and slowly add in flour mixtures, alternating with the buttermilk, until well combined. Scrape down sides as necessary.
Place springform pan on a foiled lined baking sheet and bake for 20-30 minutes, until well risen and golden brown. A tester should come out with only a few crumbs attached. Cool completely on a wire rack.