Preheat your oven to 350 degrees F. Prepare four 6-inch round cake pans by spraying each pan with non-stick spray, covering the bottom of the pan with parchment paper, and spraying the parchment paper with the non-stick spray as well. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, combine the cake flour, sugar, Jell-O, baking powder, and salt. Stir to combine.
Add the cubed butter, one piece at a time, and 1/2 cup of buttermilk to the dry ingredients. Beat on medium-low speed until light and fluffy. About 2 minutes. Scrape down the sides of the bowl.
In a small mixing bowl, whisk the egg whites and remaining 3 tablespoons of buttermilk. Add the mixture to the mixing bowl in 2 separate additions, mixing for about 30 seconds and scraping down the sides of the bowl in between each addition. Mix in the vanilla.
Divide the batter evenly between two of the four 6-inch pans (about 12 ounces of batter in each pan). Bake for about 15 minutes.
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. Level the layers before freezing, if needed.