In a medium saucepan, combine granulated sugar and water, stirring gently until the sugar is moistened. Place over medium heat and let it cook without stirring for 8-10 minutes until deep amber color.

Once the caramel has reached the desired color, immediately add in cubed butter and mix until smooth. Gradually pour in heavy cream while stirring continuously and add a pinch of salt, mix well, and set aside to cool.

Preheat your oven to 340ºF (170ºC) and line three 20 cm round cake pans with parchment paper. In a large bowl, sift together flour, cornstarch, baking powder, and baking soda.

Mix cream butter and sugar until light and fluffy, then beat in eggs one at a time. Fold in dry ingredients with sour cream, oil, and vanilla extract until a smooth batter is formed.

Divide the 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 wire racks.

Beat softened butter until fluffy. Gradually add powdered sugar and mix until incorporated. Drizzle in cooled salted caramel and vanilla until smooth.

Layer the cooled cake with buttercream and salted caramel. Crumb coat the entire cake with a thin layer of buttercream, chill for 20 minutes before frosting the final coat.

Drizzle melted salted caramel over frosted cake and sprinkle flaky sea salt on top. Let it sit for a few minutes before serving.