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 increase the heat and allow it to boil without stirring. Watch until it turns deep amber, about 10 minutes. Whisk in 1/2 cup of unsalted butter and 1/2 cup of heavy cream. Stir in 1 teaspoon of sea salt, then cool completely in the fridge.

Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans with non-stick spray. Line the bottoms with parchment paper. Sift together 2 cups of all-purpose flour, 1/4 cup of cornstarch, 1 1/2 teaspoons of baking powder, and 1/2 teaspoon of baking soda.

In a large bowl, beat 1 cup of unsalted butter and 1 1/2 cups of granulated sugar with an electric mixer on medium speed until light and fluffy, about 3-4 minutes. Add 3 large eggs, mixing well after each. Alternate adding dry ingredients and 1 cup of sour cream along with 1/4 cup of vegetable oil, starting and ending with dry ingredients.

Divide the batter between the prepared pans, smoothing the tops. Bake for 20-23 minutes, or until a toothpick comes out clean. Allow the layers to cool in the pans for 10 minutes before transferring to a wire rack.

In a medium bowl, whip 1 cup of softened butter until light and fluffy. Gradually add 4 cups of powdered sugar, 1 cup at a time, mixing on low. Mix in 1 teaspoon of vanilla extract and 1/2 cup of cooled caramel until smooth.

Once cooled, place one layer on a cake stand. Spread buttercream over the top and drizzle caramel. Place the second layer on top and apply a thin crumb coat of buttercream. Chill for 30 minutes.

Frost the cake with remaining buttercream, smoothing it out. Drizzle reserved caramel over the top and sprinkle with extra sea salt before serving.