Preheat the oven to 350 degrees F. Set out two 9-inch cake pans. Trace the bottom of the pans onto parchment paper and cut the circles out to fit the bottom of the pans exactly. Then once the parchment paper is down in the bottom, grease and flour the pans.
In the bowl of a stand mixer, cream the butter and sugar on high speed, until ultra light and fluffy. (about 3-5 minutes) Then scrape the bowl with a rubber spatula.
In a separate bowl, mix the cake flour, baking powder, baking soda, and salt.
Pour the buttermilk in a large measuring pitcher. Then whisk in the eggs, egg white, and vanilla extract.
Turn the mixer on low and alternate adding in the flour mixture and the buttermilk, until all is combined. Scrape the bowl again. Then beat again for 15-30 seconds, just until the batter is smooth. Do not over-mix the batter.
Divide and pour the cake batter equally in the prepared pans and place on the center rack in the oven. Bake 9-inch pans for 23-27 minutes. Test the cake by inserting a toothpick into the center. If it comes out clean, remove from the oven.
Cool the cake in the pans for 15 minutes, then carefully flip out onto a cooling rack and peel off the parchment paper. Cool completely.
Once cooled, use a serrated knife to cut both cakes in half to create 4 even cake layers.
Carefully wrap each cake layer in plastic. Move them onto a baking sheet and place in the freezer. Freeze the cakes for at least one hour, or until rock-hard. Once the cakes have been in the freezer for at least 30 minutes, start the frosting.
For the Frosting:
Set a large 7-8 quart sauce pot over medium heat. Clip a candy thermometer into the side of the pot. Place all the frosting ingredients, except the vanilla extract, into the saucepot. Bring to a simmer. The mixture will boil up high at first, but will settle down as it starts to caramelize. Stir regularly.
Watch the candy thermometer and continue to stir. Once the caramel mixture is medium-brown in color and reaches 235 degrees F, remove from heat. (This can take anywhere from 25-40 minutes depending on your stovetop.)
Stir in the vanilla extract. Then allow the mixture to cool for at least 15 minutes.
Use a hand mixer (or pour into a stand mixer) to beat the caramel frosting for 3-5 minutes to loosen the texture. The frosting should have a spreadable mud-like texture.
Pull the hardened cake layers out of the freezer. Unwrap and set one cake layer on a cake plate. Use a long flat spreading knife to spread approximately ½ cup of frosting over the cake layer. Unwrap and set the second cake layer on top. Press to level the cake, making sure the edges are aligned.
Spread another ½ cup of frosting over the cake. Continue stacking and frosting the remaining cakes layers until all four cake layers are stacked and leveled.
Scoop the remaining frosting over the top of the cake. Spread the frosting over the top and down the sides of the cake. Because the frosting is so dense, it can be a little tricky to cover the edges and get smooth coverage. If needed, dip the spreader in warm water to smooth out the final coating.
Chill the cake for at least 30 minutes to set the frosting.
When ready to serve, let the cake sit out at room temperature for 30-60 minutes to fully thaw, so the interior cake layers are soft and delicate. Cut and serve.
Flour Measuring – It’s very important to measure the flour properly. Stir the flour in the bag to lighten it up. Spoon the flour into measuring cups. Then level off the top.Learn how to make your own buttermilk here!