In a medium saucepan, combine 1 cup of granulated sugar with 1/4 cup of water over medium heat. Stir gently until the sugar dissolves, then let it cook without stirring until it turns a deep amber color, about 10 minutes. Remove from heat and whisk in 1/2 cup of butter and 1/2 cup of heavy cream until smooth. Allow to cool in the fridge while you prepare the rest of the Salted Caramel Cake.

Preheat your oven to 340°F (170°C) and grease three 8-inch round cake pans. In a bowl, sift together 2 cups of all-purpose flour, 1/4 cup of cornstarch, 2 tsp of baking powder, and 1/2 tsp of baking soda. In a separate large bowl, cream 3/4 cup of butter with 1 1/2 cups of granulated sugar until fluffy. Add 3 large eggs, 1 cup of sour cream, 1/2 cup of vegetable oil, and 2 tsp of vanilla extract, mixing well. Gradually incorporate the dry ingredients until combined.

Divide the batter evenly among the prepared cake pans, smoothing the tops with a spatula. Bake in the preheated oven for 20-23 minutes or until a toothpick inserted into the center comes out clean. Once baked, remove the cakes and let them cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

In a mixing bowl, beat 1 cup of softened butter using an electric mixer until creamy and fluffy, about 3-4 minutes. Gradually add 3-4 cups of sifted powdered sugar and 1 tsp of vanilla extract, mixing on low until incorporated. Slowly pour in the cooled caramel sauce, continuing to beat until the mixture is smooth and spreadable. Set the buttercream aside while you assemble the cake.

Once the cake layers are completely cooled, place one layer on a serving plate. Spread a layer of caramel buttercream on top, followed by a drizzle of the homemade caramel. Repeat with the second layer, adding more buttercream and caramel. Place the final layer on top and apply a crumb coat of buttercream around the entire cake. Chill in the fridge for 20 minutes to set before finishing the exterior.

After chilling, spread the remaining buttercream over the top and sides of the Salted Caramel Cake, smoothing it evenly with a spatula. Drizzle additional cooled caramel on top, allowing it to cascade gently over the sides. For a decorative touch, sprinkle with coarse sea salt on top. Your decadent Salted Caramel Cake is now ready to be enjoyed!