Beat the butter on high speed for about 2 minutes until it's light and fluffy, using a stand mixer fitted with the paddle attachment or a hand mixer.
Drizzle in the salted caramel sauce slowly as the mixer is running on low speed. Feel free to use store-bought caramel sauce for this (go for a variety in a jar, not a squeeze bottle...they're generally thicker and higher quality), or make your own.
Add the powdered sugar 1 cup at a time, scraping down the sides of the bowl to make sure it's fully incorporated.
Add the salt and the heavy cream, mixing on medium-high speed until the frosting becomes light and fluffy.
Trim the cakes (if necessary) to level them, using a serrated knife.
Place one cake, right side up, on a plate or cake board. Add a large dollop of frosting on top and spread it out in a thick layer to reach the edges of the cake.
Place the other cake on top of the frosting, upside down so the flat side faces up. Add the remainder of the frosting to the top of this cake and spread it over the top and down the sides of the cake, making sure to smooth it out to fill in any holes or cracks in the cake to create a very smooth edge.
Remove any excess frosting - it's fine if the cake is showing through just a little bit...this creates a "naked cake" look. Place the cake in the fridge to chill for about 45 minutes. Refrigerating the cake will cause the butter in the frosting to harden and it will create the perfect canvas for the salted caramel topping.