Preheat oven to 350F and spray a 10 inch bundt pan with non stick spray.
In a medium bowl, sift together cake flour, baking powder, baking soda, cornstarch, and salt. Set aside.
Use a mixer with a whisk attachment to cream together butter, oil, and sugar. It will be very light in color and fluffy.
Add in sour cream, lemon zest, and lemon juice. Mix until all combined. Then, add in eggs one at a time. Don't over mix. Once thoroughly combined, stop mixing.
Add ⅓ of the dry ingredients. Add in ½ of the milk. Mix until almost combined. Add in the next ⅓ of the dry ingredients and the rest of the milk. Mix until almost combined again. Add in the rest of the dry ingredients. Mix until thoroughly combined.
Take the bowl off the mixer, use a rubber spatula to scrape the bowl and make sure it is all mixed. Pour batter into bundt pan.
Bake for 30-40 minutes. The cake is done when the edges are golden brown and when a toothpick is inserted it comes out clean.
Let the cake sit in the pan for 45 minutes. Then, flip it onto a cooling rack. Allow the cake to cool completely before frosting.