Preheat oven to 325 degrees. Line the bottom of 3, 8" round cake pans with parchment paper. Butter and flour sides of pan (I use Baking PAM to do this).
Use stand mixer with paddle attachment to beat butter and sugar until lightened, about 2 minutes.
Add eggs, one at a time, mixing until well combined, and scraping down sides as needed.
Mix in lemon juice and lemon zest.
In a small bowl, stir together flour, cornstarch, salt, baking soda, and baking powder. Gradually mix into butter mixture, about 1/2 cup at a time, until well combined. Scrape down sides as needed.
Fold in sour cream until well incorporated.
Divide mixture evenly among the three prepared pans. Bake for 20-23 minutes, or until tops of cake are starting to turn golden and a toothpick inserted into center of cake comes out with just a few crumbs.
Let cool in pans for at least 5 minutes before transferring to cooling rack. Cool completely before assembling.