Preheat oven to 350°F. Line three 6-inch pans with parchment rounds and grease with non-stick baking spray.

Mix together all dry ingredients (flour, baking powder, sugar, and salt) in a stand mixer with a paddle attachment or hand mixer until fully combined.

Mix chunks of room-temperature butter slowly into the dry mix, on a low speed. Continue to mix until no large chunks of butter remain, and the mixture becomes crumbly.

Pour in egg whites, and mix on low until just incorporated. Mix in the buttermilk in two installments, on a low speed.

Add in vanilla extract and oil, and mix at a low speed until fully incorporated.

Scrape down the sides of the bowl with a spatula, then beat on medium speed for about 30 seconds. 

Divide batter evenly between the prepared cake pans. I like to use a digital kitchen scale to weigh my pans, and ensure they all have the same amount of batter. This guarantees your layers will bake to be the same height.

Bake for 30-33 minutes, or until a toothpick comes out clean. Allow the pans to cool for 10 minutes, then run a small offset spatula around perimeter of the pan to separate the cake from the pan.

Place cake layers into the freezer for 30 minutes, to accelerate the cooling process.Once the layers are fully cooled, carefully flip the pans and remove the layers.

Use a serrated knife to level the tops of the layers, and then frost as desired.

Next, make the caramel sauce. Turn stove to medium heat, and place a pot over the element.

Pour in sugar gradually, adding 1/4 cup at a time. Wait until the sugar is mostly liquified, then add the next increment of sugar.

As the sugar melts, it will gradually deepen in color.

Stir occasionally until the sugar is fully dissolved, and is an amber color.

Turn off the heat.

Mix in butter slowly (1 Tbsp at a time), then stir in cream and salt. The mixture will be very thin, but will thicken as it cools.

Pour into a separate container, then place in fridge to cool for 20 minutes.

As the caramel cools, make the caramel buttercream.

Beat the butter on a medium speed for 30 seconds with a paddle attachment or hand mixer, until smooth. Mix in the vanilla and salt on a low speed.

Slowly add in the powdered sugar, 1 cup at a time. Alternate with the heavy cream and caramel, which will help thin the frosting.

Beat on low until the ingredients are fully incorporated, and the desired consistency is reached. 

Stack and frost cake layers on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board.

Add an even layer of salted caramel buttercream between each cake layer, as well as a large swirl of caramel sauce.

Once the layers are stacked, spread a thin coat of frosting around the the cake to fully cover the cake layers.

Smooth using a bench scraper, then chill the cake in the fridge (10 minutes) or freezer (5 minutes) until the frosting is firm to the touch.

Add a second, thicker layer of frosting to the cake, and smooth using a bench scraper.

Chill the cake for an additional 10 minutes in the freezer. Once the frosting is firm to the touch, use the caramel sauce to add drips around the cake.

Cover the top of the cake with caramel sauce, and add caramel squares around the top of the cake.

Finish with a sprinkle with a flourish of sea salt on top of the cake.