Preheat your oven to 350°F (175°C). Cut out three 9-inch rounds of parchment paper to line your cake pans. Generously spray the sides and bottoms of the pans with nonstick spray, place the parchment circles into the bottom of each pan, and spray the paper again to ensure full coverage. This step is essential to prevent the cakes from sticking. Set the prepared pans aside.

In the bowl of a stand mixer fitted with a paddle attachment—or in a large bowl using a handheld mixer—combine the granulated sugar, brown sugar, flour, cocoa powder, baking soda, baking powder, and salt. Mix on low speed until all the dry ingredients are fully incorporated, breaking up any large clumps by hand if necessary.

In another bowl, whisk together the eggs, egg yolks, sour cream, milk, oil, and vanilla extract until smooth and well blended. Pour this wet mixture into the bowl with the dry ingredients and mix on low speed until just combined. Gradually add the hot water and continue mixing until the batter is fully blended and smooth—this should take about one minute. The batter will be quite thin, which is normal.

Evenly divide the batter between the three prepared pans. Bake for about 30 minutes, or until a toothpick inserted in the center of the cakes comes out clean or with a few moist crumbs. Let the cakes cool in their pans for 10 minutes before turning them out onto a wire rack. Allow the cakes to cool completely before assembling and frosting.

In the bowl of a stand mixer fitted with a paddle attachment, beat the softened butter on medium speed for about 3 minutes, or until completely smooth and creamy.

Turn off the mixer and sift the confectioners’ sugar and cocoa powder into the bowl. Turn the mixer on low and mix until the dry ingredients are fully incorporated into the butter—this will take around 2 minutes. Increase the speed to medium, then add in the vanilla extract, salt, heavy cream, and salted caramel. Beat for about 3 more minutes, until the frosting is fluffy and well blended. If the consistency is too thin, add a bit more powdered sugar. If it’s too thick, gradually mix in more heavy cream, one tablespoon at a time, until the desired texture is reached.

Using a serrated knife, carefully level the tops of the cakes by trimming any domed areas, ensuring each layer is flat and even. Place one cake layer on a cake stand or large serving plate. Spread a thin, even layer of frosting over the top, then drizzle or spread on 1/2 cup of salted caramel. Add the second cake layer and repeat the process: a thin layer of frosting followed by 1/2 cup of caramel. Place the final cake layer on top, then refrigerate the assembled cake for about 1 hour to help it set. Once chilled, frost the top and sides of the cake completely with the remaining chocolate frosting. Garnish by sprinkling flaky sea salt over the top. Slice and serve, or store the cake in the refrigerator for up to five days.