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 boil undisturbed for about 10 minutes, until it turns a deep amber color. Slowly add 1/2 cup of butter and 1 cup of heavy cream, whisking continuously to prevent splattering. Remove from heat and let cool in the refrigerator while you prepare the cake.

In a large mixing bowl, sift together 2 cups of all-purpose flour, 2 tablespoons of cornstarch, 1 tablespoon of baking powder, and a pinch of salt. In another bowl, beat 1/2 cup of softened butter with 1 cup of granulated sugar until light and fluffy, about 3-4 minutes. Add 3 eggs, one at a time, then mix in the dry ingredients, 1 cup of sour cream, 1/2 cup of vegetable oil, and 2 teaspoons of vanilla extract until just combined.

Preheat your oven to 170ºC (340ºF) and grease two 9-inch round cake pans. Pour the prepared cake batter evenly into the pans, smoothing the tops with a spatula. Bake for 20-23 minutes, or until a toothpick inserted into the center comes out clean. Once done, remove the pans from the oven and let them cool on a wire rack for 10 minutes before removing the cakes from the pans.

In a large mixing bowl, beat 1 cup of softened butter until creamy. Gradually mix in 4 cups of powdered sugar, a pinch of salt, and 1 teaspoon of vanilla extract until well blended. Add 1/2 cup of the cooled salted caramel sauce and beat until light and fluffy, about 5 minutes.

Once the cakes are completely cooled, place one layer on a serving plate. Spread a generous layer of buttercream on top, then drizzle salted caramel over it. Place the second cake layer on top, then cover the entire cake with a crumb coat of buttercream, refrigerating for about 20 minutes to set. Decorate with the remaining buttercream and drizzle more salted caramel sauce on top for a stunning finish.