Preheat oven to 350F (conventional)
Sift flour, cornstarch, baking powder, baking soda and salt into a bowl and mix. Set aside
In a separate bowl, whisk the sugar, oil and butter for 1-2 minutes with a hand mix until the mixture starts to change color. Add the eggs, egg white and vanilla, and whisk for another 1-2 minutes
Add a third of the buttermilk to the wet ingredients and whisk to combine. Add a third of the dry ingredients and fold with a rubber spatula gently until mixed in. Repeat, adding the buttermilk and dry ingredients in thirds and combining until everything is mixed together
Prep a bundt pan by brushing generously with oil and dusting with granulated sugar. Pour the batter into the pan
Bake for ~30 minutes (exact time varies), until a toothpick comes out with moist crumbs attached
Let the cake rest in the pan for 10-15 min, then carefully turn it out
Dust with powdered sugar and serve!
