Preheat your oven to 350°F (conventional).

Sift together flour, cornstarch, baking powder, baking soda, and salt into a bowl and mix. Set aside.

In a separate bowl, whisk together sugar, oil, and melted butter for 1-2 minutes until it starts to change color. Add the eggs, egg white, and vanilla, then whisk for another 1-2 minutes.

Add a third of the buttermilk to the wet ingredients and whisk to combine. Then add a third of the dry ingredients and gently fold with a rubber spatula until mixed. Repeat until fully combined.

Prep a bundt pan by generously brushing with oil and dusting with granulated sugar. Pour the batter into the pan.

Bake for approximately 30 minutes, or until a toothpick comes out with moist crumbs attached.

Let the cake rest in the pan for 10-15 minutes, then carefully turn it out onto a serving dish.

Dust with powdered sugar and serve!