Preheat your oven to 177 °C (175 C). Grease three 8-inch round cake pans, line the bottoms with parchment paper rounds, and dust lightly with cocoa powder.

In a large stand mixer bowl, whisk together the all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and fine sea salt until evenly blended.

Add the buttermilk, vegetable oil, eggs, and 1 teaspoon of vanilla extract. Beat on medium-low speed for 2 minutes until fully combined. Reduce speed to low and slowly stream in the hot brewed coffee until a smooth, thin batter forms.

Divide the batter evenly among the three prepared pans. Bake for 28 to 32 minutes, or until a toothpick inserted into the center emerges with a few moist crumbs. Cool in pans for 15 minutes, then invert onto wire racks to cool completely.

Prepare the caramel by heating 240 ml of sugar in a heavy-bottomed saucepan over medium heat, swirling gently until melted into a clear amber syrup. Whisk in 90 ml of cubed butter until incorporated, then slowly pour in 120 ml warm heavy cream. Simmer for 1 minute, remove from heat, stir in flaky sea salt, and let cool completely until thick and spreadable.

Make the chocolate buttercream by beating 240 ml softened butter on medium-high speed for 3 minutes until pale and creamy. Add powdered sugar, 120 ml cocoa powder, melted cooled chocolate, remaining 1 teaspoon vanilla, and 80 ml heavy cream. Beat on low until combined, then increase to medium-high for 2 minutes until fluffy.

Assemble the cake by placing the first sponge layer on a cake board. Pipe a sturdy rim of chocolate buttercream around the top perimeter to form a dam, then spread 80 ml cooled salted caramel into the center. Top with the second layer and repeat. Place the final layer on top, frost the exterior with remaining buttercream, and crown with a drizzle of leftover salted caramel.