Make the Salted Caramel: In a medium saucepan, combine 1 cup of granulated sugar with 1/4 cup of water. Stir until boiling and turns amber, about 10 minutes. Remove from heat, add 6 tablespoons of unsalted butter, and whisk in 1/2 cup of heavy cream.

Prepare Cake Batter: Preheat oven to 170ºC (340ºF) and grease three 8-inch cake pans. Sift together 2 cups of flour, 1 tablespoon of baking powder, 1/2 teaspoon of baking soda, and salt. Cream together 1/2 cup of unsalted butter and 1 1/2 cups of granulated sugar. Add eggs one at a time, alternating with dry ingredients, 1 cup of sour cream, 1/4 cup of vegetable oil, and 2 teaspoons of vanilla extract.

Bake the Cake: Divide batter among prepared pans and bake for 20-23 minutes until a toothpick comes out clean. Cool for 10 minutes before transferring to wire racks.

Make Buttercream: Beat 1 cup of unsalted butter until light, then gradually add 4 cups of sifted powdered sugar and 1/2 cup of the cooled caramel until smooth.

Assemble the Cake: Place the first layer on a plate, spread buttercream, and drizzle with caramel. Repeat with second layer and place final layer on top. Apply a crumb coat and chill for 20 minutes.

Finish Frosting the Cake: Apply a thicker layer of buttercream and drizzle remaining salted caramel over the top, finishing with a sprinkle of sea salt.