Preheat oven to 325 degrees. Line three, 8″ round pans with parchment paper. Flour and butter sides of pan (I use baking PAM to do this).
In large mixing bowl, whisk together flour, cornstarch, salt, baking soda and baking powder.
In base of a stand mixer, whisk together sugar and lemon zest. Add softened butter and beat on medium-high speed until lightened and fluffy, about 1.5 minutes. Then, beat in eggs, one at a time, scraping down sides as needed.
Stir together oil and lemon juice in a small bowl. Then, alternate mixing in flour mixture and oil mixture in about 3 additions each. After each addition, start on low speed and gradually increase to medium-high for a few seconds. Once all flour and oil has been added, beat on medium-high until well combined. Finally, stir in sour cream until just combined.
Divide batter among the three prepared pans and bake for 27-30 minutes, or until tops are golden and toothpick in center of cake comes out with just a few crumbs. Do not open oven during baking time.
Let cakes cool in pans for at least 10 minutes before removing. Cool completely before frosting.