Preheat your oven to 325 F. Spray three 8-inch round cake pans with nonstick spray, line the bottom with parchment paper and spray again. Set aside.
In a medium bowl, whisk the cake flour, baking powder, lemon zest, and salt. Sete aside.
Using a stand mixer fitted with the paddle attachment, beat butter, sugar, and Jell-O. Beat on medium-high speed for about 3 minutes until light in color and fluffy.
With the mixer on medium speed, add the egg whites, one at a time, scraping down the sides of the bowl between additions. Continue to beat on medium-high speed for 2 minutes until smooth. Add the vanilla and stir to combine.
With the mixer on low speed, add in intervals, blending after each addition, ⅓ dry ingredients, ½ buttermilk, ⅓ dry ingredients, ½ buttermilk, ⅓ dry ingredients
Scrape down the bowl and mix for another 20 to 30 seconds.
Divide the batter evenly between the pans (16 ounces in each). Bake for 23 to 25 minutes, or until a toothpick inserted in the center of the cake comes out with just a few moist crumbs on it.
Remove the cake pans from the oven and let the layers cool in pans for about 15 minutes before inverting onto cooling rack to cool completely.*