Make Salted Caramel: In a medium saucepan over medium heat, combine granulated sugar and water. Stir until sugar dissolves, then let it cook undisturbed to a deep amber color (about 10 minutes). Remove from heat, stir in butter, then whisk in heavy cream. Mix in salt and set aside to cool.

Prepare Cake Layers: Preheat oven to 170ºC (340ºF). Grease three 20 cm (8-inch) round cake pans and line bottoms with parchment paper. Sift together flour, cornstarch, baking powder, baking soda, and salt. In another bowl, beat butter and sugar until fluffy. Add eggs, then mix in sour cream, oil, and vanilla until combined.

Mix and Bake: Gently fold dry mixture into wet ingredients until just combined. Divide batter among prepared pans and smooth tops. Bake for 20-23 minutes, or until a toothpick comes out clean. Cool in pans for 10 minutes before transferring to a wire rack.

Make Caramel Buttercream: Beat room temperature butter until fluffy (about 3 minutes). Gradually add powdered sugar, beating on low then high until blended. Pour in cooled caramel sauce and vanilla, continue beating until smooth. Add heavy cream if too thick.

Assemble Cake: Once cake layers are cooled, place one layer on a serving plate. Spread with caramel buttercream and drizzle with salted caramel sauce. Repeat with second layer and top with final layer. Apply a thin crumb coat over the entire cake and refrigerate for 30 minutes.

Final Decoration: Spread a thicker layer of caramel buttercream over the cake. Decorate the top with leftover salted caramel sauce and sprinkle with flaked sea salt. Slice and serve.