Preheat your oven to 325 degrees F. Spray three 8-inch cake pans with nonstick spray, line the bottoms with parchment paper and spray again. Set aside.
In a medium bowl, whisk the cake flour, baking powder and salt.
In the bowl of an electric mixer fitted with the paddle attachment, combine the butter, sugar, Jell-O and zest. Beat on medium-high speed until light and fluffy, about 2 minutes.
Add the egg whites, in a few additions, beating well and scraping down the sides of the bowl between each addition. Add the vanilla and stir to combine.
With the mixer on low speed, alternately add the flour mixture and the buttermilk, starting and ending with the flour mixture. Mix on low, just until flour is combined.
5. Divide the batter evenly between the pans and bake for 25-30 minutes.
6. Let cool in the pans for about 10 minutes and then invert onto wire cooling wraps to cool completely. Cakes can be individually wrapped in plastic wrapped and frozen for a week. If you'd like to freeze them longer than one week, wrap each cake layer in tin foil as well.