Preheat the oven to 350°F and have ready 2 greased 9" baking pans, lined with parchment paper on the bottom.
Sift the cake flour, baking powder, baking soda, and salt over a medium-sized mixing bowl. Set aside until ready to use.
Add the butter and sugar to a large bowl and using a handheld mixer, mix until fluffy and pale yellow in color (about 3-5 minutes). You could also use a stand mixer fitted with a paddle attachment for this task.
Add the eggs, one at a time, mixing well after each addition. Add the vanilla and mix again. Scrape down the sides and the bottom of the bowl using a silicone spatula and mix once more.
With the mixer on low, add the dry ingredients in 3 rounds, alternating with the buttermilk (adding it in 2 rounds). Scrape the sides and the bottom of the bowl, and mix once again.
Divide the batter among the prepared pans, and smooth over the top. Bake for 30-35 minutes. Set the cake aside to cool completely on a wire rack before icing.
To Make the Icing:
For the frosting, you will need a large mixing bowl and a handheld mixer or a stand mixer fitted with a paddle attachment. Add the butter and 1 cup of caramel sauce to the bowl and mix on medium-high speed for 2-4 minutes, until very smooth.
Add the powdered sugar, vanilla, and salt and begin mixing on low. Once the sugar is mostly incorporated, you can begin mixing on medium-high. Mix for 2 minutes more, until light and fluffy.
Level out your cake layers using a long, serrated knife. Place one of the cake layers on a cake stand or serving dish, and top with a large dollop of frosting. Spread the frosting to the edges and repeat with the second layer. Use the remaining frosting to frost the top and sides of the cake. Drizzle the top with the remaining ⅓ cup of caramel sauce and sprinkle with Kosher salt, if desired. Serve, and enjoy!
Notes
HOW TO KNOW WHEN THE CAKE IS DONE:
The cake is ready when the center of it springs back ever so slightly after a gentle poke or a toothpick inserted in the center comes out with only a few crumbs clinging to it.