Preheat oven to 350℉, grease and parchment line the bottom of two six-inch cake pans.
Whisk together flour, cornstarch, baking powder and salt and set aside.
1 ½ cups all-purpose flour, ¼ cup cornstarch, 2 teaspoons baking powder, ¼ teaspoon kosher salt
Using fingertips, mix together sugar and lemon zest until pale yellow and very fragrant.
¾ cup granulated sugar, ¼ cup lemon zest
Cream together the butter, sugar, and oil until pale in color, about 5 minutes. Add the egg and vanilla and beat again for one minute. Scrape down the sides.
6 tablespoons unsalted butter, ⅓ cup vegetable oil, 1 large egg, 2 teaspoons vanilla paste or extract
Whisk together the sour cream and whole milk.
½ cup full-fat sour cream, ½ cup whole milk
With the mixer running on low speed, add half of the flour mixture, then half of the milk mixture. Scrape down the sides and repeat until the batter is completely moistened.
Divide the batter evenly between the two cake pans then bake in the center rack for 28-32 minutes or until a toothpick inserted into the center comes out clean with no wet batter.
Cool in the pan for 10 minutes, then remove the cake from the pan and finish cooling for another 30 minutes, or until no longer warm to the touch.
If storage is needed, wrap the cake layers tightly in plastic wrap and store at room temperature for 24 hours, or in the freezer if storage is needed for longer.