Step 1: Preheat your oven to 350°F (175°C). This ensures your cake will bake evenly.
Step 2: Grease two 9-inch round cake pans with butter or cooking spray and line the bottoms with parchment paper for easy removal.
Step 3: In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and kosher salt until well combined.
Step 4: In another bowl, use an electric mixer to cream the softened butter and sugar together until light and fluffy, about 3-5 minutes.
Step 5: Add the eggs, one at a time, mixing well after each addition. Then, mix in the vanilla extract until fully incorporated.
Step 6: Gradually add the dry mixture to the butter mixture, alternating with the buttermilk. Begin and end with the flour mixture, mixing just until combined.
Step 7: Divide the batter evenly between the prepared cake pans. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean.
Step 8: Once baked, allow the cakes to cool in the pans for about 10 minutes, then transfer them to a cooling rack to cool completely.
Step 9: In a mixing bowl, cream together the softened butter and shortening. Gradually add the confectioners sugar, mixing well. Slowly add the buttermilk and sea salt, beating until smooth and creamy.
Step 10: Once the cakes are completely cooled, place one layer on a serving plate. Spread a generous layer of salted caramel buttercream on top. Place the second layer on top and frost the top and sides of the cake.
Step 11: Slice the cake and serve it to family and friends. Enjoy the delightful combination of the rich butter cake and the sweet yet salty frosting!