In a medium saucepan, combine granulated sugar and a splash of water over medium heat. Stir gently until the mixture begins to bubble and turns a rich golden brown, approximately 8-10 minutes. Carefully stir in butter and heavy cream, watching out for vigorous bubbling. Once combined, remove from heat and let the salted caramel cool as it thickens. Transfer to the fridge to set while you move on to the cake.

In a large mixing bowl, sift together all-purpose flour, cornstarch, baking powder, baking soda, and salt. This step is crucial for aerating the flour, ensuring your Salted Caramel Cake has a light and fluffy texture. Once sifted, whisk the dry ingredients gently to combine, then set aside, allowing the flavors to meld as you prepare the wet ingredients.

In another bowl, cream together room temperature butter and granulated sugar until the mixture is light and fluffy, about 3-4 minutes. Gradually add in the eggs, one at a time, mixing well after each addition. Next, incorporate the sour cream, vegetable oil, and vanilla extract. Stir in the dry ingredient mixture in three parts, alternating with the wet components for a smooth batter, ensuring everything is fully combined and well incorporated.

Preheat your oven to 170ºC (340ºF). Prepare three 20 cm round cake pans by lining the bottoms with parchment paper and greasing the sides. Divide the batter evenly among the pans, smoothing the tops with a spatula. Bake in the preheated oven for 20-23 minutes, or until a cake tester comes out clean and the edges are golden brown. Allow the layers to cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

In a large mixing bowl, beat softened butter until creamy and smooth. Gradually add in the sifted powdered sugar and cooled salted caramel, mixing on low speed initially to combine, then increase speed to medium until the buttercream is fluffy and well blended. Taste and adjust the vanilla extract as desired, ensuring the buttercream is both sweet and infused with delicious caramel flavor for frosting the layers.

To assemble your Salted Caramel Cake, place one layer of the cooled cake on a serving plate. Generously spread a layer of buttery caramel buttercream on top and drizzle some of the cooled salted caramel over it. Repeat this process with the second cake layer, adding more frosting and caramel. After placing the final layer, apply a crumb coat of buttercream around the sides and top of the cake. Chill for 30 minutes to set.

Once chilled, take the cake out and frost the top and sides with the remaining buttercream, smoothing it out for a clean finish. Decorate with drizzles of the leftover salted caramel, allowing it to cascade down the sides, and sprinkle flaky salt on top for that perfect contrast. Your Heavenly Salted Caramel Cake is now ready to impress at your next gathering!