Preheat oven to 350°(F). Cut out three 9-inch round segments of parchment paper to line your cake pans with. Spray each pan generously - sides and bottom - with nonstick cooking spray, then place the parchment paper cut out in the bottom of the pans and spray again. It's important to make sure every bit of pan and paper are sprayed so your cakes don't get stuck. Set pans aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld electric mixer, combine both sugars, flour, cocoa powder, baking soda, baking powder and salt; mix on low until dry ingredients are thoroughly combined. Use your hands to break up any large clumps, if needed.
In a separate bowl combine the eggs, egg yolks, sour cream, milk, oil and vanilla extract; mix until completely combined. Pour mixture into the dry ingredients and beat on low until just incorporated. Pour in hot water and continue mixing until completely combined; about 1 minute. The batter will quite thin.
Divide batter evenly among prepared pans. Bake in preheated oven for 30 minutes, or until a wooden toothpick or cake tester inserted in the center of a cake comes out clean or with just a few moist crumbs attached. Cool cakes for 10 minutes in the pan before removing from pans and transferring to a cooling rack; cool cakes completely before frosting.
For the chocolate frosting:
In a stand mixer fitted with the paddle attachment, cream the softened butter on medium-speed until completely smooth; about 3 minutes.
Turn the mixer off and sift the powdered sugar and cocoa into the mixing bowl. Turn the mixer on the lowest speed and mix until the sugar/cocoa have been absorbed by the butter; about 2 minutes. Increase mixer speed to medium; add in vanilla extract, salt, heavy cream and salted caramel; beat for 3 minutes. If your frosting appears a little too thin, add a little more confectioners' sugar; If your frosting needs to be thinner, add additional heavy cream, 1 tablespoon at a time.
Using a serrated knife, carefully trim the raised top of each cake, making each one an even, level surface. Transfer 1 layer to a large plate or cake stand. Spread a thin layer of frosting on top, then add a 1/2 cup of caramel; top with another cake layer, and repeat, thinly spread it with a layer of frosting, then adding a 1/2 cup of caramel. Top final cake layer and place cake in the fridge to set for 1 hour. Once set, finishing frosting the top and sides of the cake. Sprinkle with sea salt. Slice and serve, or keep refrigerated for up to 5 days.
Cake may be baked up to 5 days in advance and stored in the fridge. Unfrosted cakes may be wrapped and frozen for up to 2 months. For best results, follow the recipe as written.