Step 1: Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans. Line the bottoms with parchment paper for easy release.
Step 2: In a large bowl, whisk together the all-purpose flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt. Make sure there are no lumps.
Step 3: In a separate medium bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla extract until well combined.
Step 4: Gradually add the wet ingredients to the dry ingredients, mixing on low speed until just combined. Do not overmix! Stir in the hot coffee (or hot water if you prefer) until the batter is smooth. The batter will be thin, but this is normal.
Step 5: Divide the batter evenly among the three prepared cake pans. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs.
Step 6: Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely. Ensure they are fully cooled before frosting to prevent melting.
Step 1: In a large heatproof bowl set over a saucepan of simmering water (a double boiler), melt the unsweetened chocolate and semi-sweet chocolate, stirring until smooth. Remove from heat and let cool slightly.
Step 2: In a stand mixer fitted with the paddle attachment, beat the softened unsalted butter on medium speed until creamy.
Step 3: Gradually add the powdered sugar, one cup at a time, beating until fully incorporated.
Step 4: Beat in the cooled melted chocolate.
Step 5: With the mixer on low, slowly add the heavy cream and vanilla extract. Increase speed to medium-high and beat for 3-5 minutes until the frosting is light, fluffy, and spreadable. If it's too thick, add a bit more cream; if too thin, add more powdered sugar.
Step 1: In a medium heavy-bottomed saucepan, combine the granulated sugar and 1/4 cup water. Stir gently to moisten all the sugar.
Step 2: Cook over medium heat without stirring until the sugar dissolves and the syrup begins to boil. Continue to cook without stirring, occasionally swirling the pan, until the caramel turns a deep amber color. This usually takes 8-10 minutes. Watch it carefully, as it can burn quickly!
Step 3: Carefully and slowly pour in the warm heavy cream (it will bubble vigorously). Whisk constantly until smooth. Remove from heat.
Step 4: Stir in the butter and sea salt until fully incorporated.
Step 5: Pour the caramel into a heatproof container or jar and let it cool completely. It will thicken as it cools.
Step 1: Place one cooled cake layer on a serving plate or cake stand. Spread about 1 cup of chocolate fudge frosting evenly over the top.
Step 2: Place the second cake layer on top and repeat with another cup of frosting.
Step 3: Top with the third cake layer. Spread a thin layer of frosting over the entire cake (top and sides) for a crumb coat. This helps seal in any loose crumbs. Refrigerate for 15-20 minutes to set.
Step 4: Apply the remaining chocolate fudge frosting evenly over the top and sides of the cake, smoothing it with an offset spatula or bench scraper.
Step 5: Once frosted, gently warm the cooled salted caramel drizzle until it's pourable but not hot. Drizzle generously over the top of the cake, allowing some to drip down the sides.
Step 6: Optionally, garnish with chocolate shavings, additional sea salt, or even a few chocolate curls for an extra touch of elegance. Slice and serve this magnificent cake.