Step 1: Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans, or line the bottoms with parchment paper.
Step 2: In a large bowl, whisk together the 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.
Step 5: Slowly pour in the hot coffee, mixing until the batter is smooth. The batter will be thin, this is normal.
Step 6: Divide the batter evenly among the three prepared cake pans.
Step 7: Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs.
Step 8: Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.
Step 9: For the Fudge Frosting: In a large heatproof bowl set over a saucepan of simmering water (bain-marie), combine the unsweetened chocolate, unsalted butter, and powdered sugar. Stir occasionally until melted and smooth. Remove from heat.
Step 10: Whisk in the heavy cream and vanilla extract until the mixture is smooth and glossy.
Step 11: Allow the fudge frosting to cool slightly at room temperature, then chill in the refrigerator for about 30-45 minutes, stirring occasionally, until it reaches a spreadable consistency.
Step 12: For the Salted Caramel Drizzle: In a heavy-bottomed saucepan, combine 1 cup granulated sugar and 1/4 cup water. Stir gently to dissolve the sugar.
Step 13: Cook over medium heat without stirring until the sugar dissolves and the mixture begins to boil. Once boiling, stop stirring.
Step 14: Continue to cook, swirling the pan occasionally, until the syrup turns a rich amber color (8-10 minutes). Watch it carefully.
Step 15: Carefully and slowly pour in the warm heavy cream while whisking constantly. The mixture will bubble vigorously. Continue whisking until smooth.
Step 16: Remove from heat and stir in the unsalted butter and sea salt until fully incorporated and smooth.
Step 17: Let the caramel cool completely to room temperature. It will thicken as it cools.
Step 18: Assembling: Once the cake layers are completely cool, place one layer on your serving plate. Spread about 1/3 of the fudge frosting evenly over the top.
Step 19: Place the second cake layer on top and repeat with another 1/3 of the frosting.
Step 20: Top with the third cake layer and spread the remaining frosting over the top and sides of the cake, smoothing it out with an offset spatula.
Step 21: Drizzle the cooled salted caramel generously over the top of the cake, allowing it to drip down the sides.
Step 22: For best results, chill the cake for at least 30 minutes before serving to allow the frosting and caramel to set.