Once the caramel reaches a golden color, reduce heat to medium. Stir in the butter until fully melted. Slowly add the heavy cream, a little at a time, stirring continuously. The mixture will bubble, so be cautious. Stir until smooth and well combined.

Turn off the heat and stir in a pinch of salt. Pour the sauce into a shallow bowl to cool quickly, then refrigerate for 30-60 minutes until it reaches room temperature.

Preheat the oven to 170°C (340°F) and prepare three 20 cm (8 in) round cake pans by lining them with parchment paper.

Sift together flour, cornstarch, baking powder, baking soda, and salt. Set aside.

In a large mixing bowl, beat the butter and sugar together on high speed for about 3 minutes, until light and fluffy.

Add eggs, two at a time, mixing well between each addition.

Add half of the dry ingredients to the wet mixture, mixing on low speed until just combined.

Add sour cream, vegetable oil, and vanilla extract, and mix until combined. Then add the remaining dry ingredients and mix until just combined.

Use a rubber spatula to gently fold the batter to ensure everything is fully incorporated.

Divide the batter evenly between the prepared pans. Bake for 19-22 minutes or until a toothpick inserted into the center comes out clean. The cakes should remain light and not overly browned.

Allow the cakes to cool in the pans for 10 minutes before removing them and letting them cool completely on a wire rack.

Sift the powdered sugar and set it aside.

In a stand mixer or with a hand mixer, beat the softened butter on medium-high speed for 4 minutes until creamy. Scrape down the sides of the bowl and continue mixing for another 2 minutes.

Gradually add the powdered sugar, vanilla extract, and caramel in two parts, mixing well after each addition.

Scrape down the sides and mix again on low speed for a final 1 minute.

Place the first cake layer on a cake board or serving dish. Spread a generous layer of caramel buttercream over the top with an offset spatula. Drizzle 2-3 tablespoons of salted caramel sauce over the buttercream, ensuring to leave the edges clear.

Add the second cake layer, repeating the process with buttercream and caramel sauce. Place the final cake layer on top, then apply a thin crumb coat of buttercream around the entire cake.

Refrigerate the cake for 20 minutes to firm up the crumb coat.

Once firm, apply the remaining buttercream to the entire cake and smooth it out. Drizzle with the remaining salted caramel sauce on top, and optionally swirl it into the buttercream for a decorative effect.