In a medium saucepan, combine 1 cup of granulated sugar and 1/4 cup of water over medium heat. Stir gently until the sugar dissolves, then let it cook undisturbed until it turns a deep amber color, about 8–10 minutes. Remove from heat and carefully add 6 tablespoons of butter and 1/2 cup of heavy cream, stirring constantly to combine. Allow the caramel to cool in the refrigerator while preparing the cake layers.

Preheat your oven to 340°F (170°C) and grease three 8-inch round cake pans. In a large bowl, sift together 2 cups of all-purpose flour, 1 1/2 teaspoons of baking powder, 1 teaspoon of baking soda, and 1 tablespoon of cornstarch. In another bowl, beat 1 cup of granulated sugar with 1/2 cup of softened butter until light and fluffy. Add in 2 large eggs, 1 cup of sour cream, and 1/4 cup of vegetable oil, mixing until well blended before gradually incorporating the dry ingredients.

Divide the cake batter evenly among the prepared cake pans and smooth the tops. Bake in the preheated oven for 20–23 minutes, or until a toothpick inserted in the center comes out clean. Once baked, let the cake layers cool in the pans for 10 minutes, then transfer to a wire rack to cool completely before frosting.

In a mixing bowl, beat 1 cup of softened butter on medium speed until creamy, about 2–3 minutes. Gradually add in 4 cups of powdered sugar and 1 teaspoon of vanilla extract, mixing until combined. Scrape down the sides and add in 1/2 cup of the cooled salted caramel, blending on low until fully incorporated. Beat on high until the buttercream is light and fluffy, about 4–5 minutes.

Place one cooled cake layer on a serving plate and spread a generous layer of salted caramel buttercream on top. Add a second layer of cake and repeat the process, leaving the top layer bare for now. Apply a thin crumb coat of buttercream around the entire cake to seal in crumbs, then refrigerate for 30 minutes to set.

Remove the cake from the fridge and apply a thick layer of the remaining salted caramel buttercream over the top and sides. Use a spatula or cake scraper to create a smooth finish. Drizzle additional salted caramel sauce over the top and sprinkle a pinch of sea salt on top.