• Set the oven to 350°F. Prepare three 8-inch round cake pans by greasing and dusting with flour, or use a nonstick baking spray. Place a sheet of parchment paper in each pan, then lightly grease and flour them once more.

Make the cake batter.

• In a large bowl, use a mixer to blend the flour, sugar, cocoa powder, baking soda, baking powder, and salt. In a separate, smaller bowl, whisk the eggs, buttermilk, coffee, oil, and vanilla extract together. Pour the wet mixture into the dry ingredients and mix on low speed for about 2 minutes, until everything is just combined. The batter will be fairly thin.
• Pour the batter evenly into the three prepared pans. Bake for 30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Let the cakes cool in their pans for 20 minutes, then transfer them to cooling racks to cool completely before adding frosting.

Make the salted caramel sauce.

• In a 3-quart saucepan over medium heat, combine water, sugar, and salt. Stir gently with a rubber spatula as the mixture heats and begins to boil, which should take about 5 minutes. Once it reaches a boil, continue boiling for another 6 to 7 minutes until the sugar mixture turns a rich amber color. When the color is just right, carefully add the heavy cream and lower the heat slightly to the upper end of medium-low.
• The mixture may seem thick and clumpy at first; keep stirring, and it will gradually become smooth and silky. Maintain a gentle boil for 3 to 5 minutes to let the sauce thicken, or use an instant-read thermometer to check for 230°F. The sauce may still look thin but will thicken as it cools. Set it aside to cool fully.

Make the frosting.

• In a large bowl, use an electric mixer with the paddle attachment to cream together the butter and half of the homemade salted caramel sauce. Beat for 2 to 3 minutes until fully blended. Gradually add the confectioners' sugar, one cup at a time, mixing thoroughly after each addition. Add the vanilla extract and a pinch of salt, then mix on high speed for one minute. If the frosting seems too dry, add a tablespoon or two of heavy cream and mix until smooth.

Assemble the cake.

• If needed, level the cake layers. Spread 1 cup of frosting over the bottom layer and drizzle with 1–2 tablespoons of the reserved salted caramel. Place the second layer on top, then add more frosting and caramel. For the final layer, place it top side down and coat the top with a thin layer of frosting. Chill the cake for 30 minutes to set. Once set, use the remaining frosting to fully cover the top and sides. Drizzle the remaining caramel down the sides for a drip effect, and pipe the rest of the frosting as decoration.