Preheat your oven to 350°F. Cut three 9-inch round parchment paper circles to line the cake pans. Generously spray the sides and bottoms of each pan with nonstick cooking spray, place the parchment circles inside, then spray the parchment again to ensure nothing sticks. Set the pans aside. In a stand mixer fitted with the paddle attachment, or using a handheld mixer in a large bowl, combine both sugars, flour, cocoa powder, baking soda, baking powder, and salt. Mix on low speed until all dry ingredients are thoroughly blended. Break up any large clumps by hand if necessary. In a separate bowl, whisk together the eggs, egg yolks, sour cream, milk, oil, and vanilla extract until fully combined. Pour the wet ingredients into the dry mixture and beat on low until just combined. Add the hot water and mix on low for about one minute until fully incorporated; the batter will be thin. Divide the batter evenly among the prepared pans and bake for about 30 minutes or until a toothpick inserted into the center comes out clean or with a few moist crumbs. Let cakes cool in pans for 10 minutes before transferring to a wire rack to cool completely before frosting.

In a stand mixer with the paddle attachment, beat the softened butter on medium speed until smooth and creamy, about 3 minutes. Turn the mixer off and sift in the powdered sugar and cocoa powder. Mix on the lowest speed until the sugar and cocoa are absorbed into the butter, about 2 minutes. Increase the speed to medium, then add vanilla extract, salt, heavy cream, and salted caramel sauce. Beat for 3 minutes until fluffy. If the frosting is too thin, add more powdered sugar; if it’s too thick, add additional heavy cream, one tablespoon at a time, until desired consistency is reached.

Using a serrated knife, level the tops of the cakes by trimming any domed areas. Place one cake layer on a serving plate or cake stand. Spread a thin layer of frosting on top, then drizzle ½ cup of caramel sauce over it. Repeat with the next cake layer, frosting, and caramel. Place the final cake layer on top, then chill the cake in the fridge for 1 hour to set. After chilling, frost the top and sides completely with the remaining frosting. Finish by sprinkling sea salt over the cake. Slice and serve, or store refrigerated for up to 5 days.