In a medium saucepan over medium heat, combine granulated sugar and water. Stir gently until the sugar dissolves and turns a golden brown color, which takes about 8-10 minutes. Remove from heat and carefully whisk in the butter, followed by the heavy cream. Set aside to cool.

Preheat your oven to 340°F (170°C). In a large mixing bowl, cream together sugar and butter for about 3-5 minutes until light and fluffy. Add the eggs one at a time, ensuring each incorporates fully. Gradually mix in the dry ingredients, alternating with the sour cream, vegetable oil, and vanilla extract, stirring until just combined.

Divide the cake batter evenly among three lined 20 cm round cake pans. Bake in the preheated oven for 20-23 minutes or until a toothpick inserted into the center comes out clean. Once baked, let them cool on a wire rack for about 10 minutes before carefully removing the cakes from the pans.

In a mixing bowl, beat room-temperature butter on medium speed until creamy, about 2-3 minutes. Gradually add in powdered sugar, followed by the vanilla extract and cooled salted caramel. Continue mixing for another 3-5 minutes until light and fluffy.

Place one cake layer on a serving plate, spreading a layer of buttercream and then pouring some cooled salted caramel on top. Repeat with the second layer. Apply a thin crumb coat of buttercream on the outside and chill for about 30 minutes.

Once chilled, cover the entire cake with a thick layer of buttercream, drizzle additional salted caramel over the top, and finish with a sprinkle of sea salt.