In a medium saucepan, combine granulated sugar and water, stirring gently until the sugar dissolves. Place over medium heat and cook for 8-10 minutes without stirring, until the mixture turns a deep amber color. Remove from heat, then carefully whisk in unsalted butter and heavy cream until smooth. Let the salted caramel cool to room temperature, then refrigerate until it thickens, about 30 minutes.

In a large mixing bowl, cream together granulated sugar and vegetable oil until light and fluffy. Add room temperature eggs, one at a time, mixing well after each addition, then blend in sour cream and vanilla extract. In another bowl, whisk together all-purpose flour, cornstarch, baking powder, and baking soda. Gradually add the dry ingredients to the wet mixture, alternating with the remaining buttermilk for a smooth, pourable batter.

Preheat your oven to 340°F (170°C) and grease two 9-inch round cake pans with cooking spray or butter. Divide the cake batter evenly between the pans and smooth the tops with a spatula. Bake for 20-23 minutes, or until a toothpick inserted in the center comes out clean. Once done, remove the pans from the oven and allow the cake layers to cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

While the cake layers cool, make the salted caramel buttercream. In a large mixing bowl, beat room temperature unsalted butter until creamy. Gradually add powdered sugar, mixing on low speed until incorporated. Pour in 1/2 cup of the previously made salted caramel, adding more to taste, along with a pinch of flaky sea salt, and beat until the mixture is light and fluffy, about 2-3 minutes.

Once the cake layers are completely cooled, place one layer on a serving plate. Spread a generous layer of salted caramel buttercream on top, followed by a drizzle of salted caramel sauce. Gently place the second cake layer on top, then frost the entire cake with the remaining buttercream. Create swirls or peaks for a beautiful finish, and drizzle the top with more salted caramel and a sprinkling of flaky sea salt.

For the best experience, let the assembled salted caramel cake chill for 30 minutes before slicing. This allows the layers to set and makes for cleaner slices. Serve with an extra drizzle of caramel and a sprinkle of sea salt on each slice.