Prep the pans and oven: Heat oven to 350°F (175°C). Grease two 8-inch round pans, line bottoms with parchment, and lightly flour the sides.
This prevents sticking and gives clean edges.
Whisk dry ingredients: In a large bowl, whisk 2 cups (250 g) flour, 1 cup (200 g) granulated sugar, 3/4 cup (150 g) light brown sugar, 3/4 cup (75 g) Dutch-process cocoa, 1 1/2 tsp baking powder, 1 1/2 tsp baking soda, and 1 tsp fine sea salt until even.
Combine wet ingredients: In a separate bowl, whisk 2 large eggs, 1 cup (240 ml) buttermilk, 1/2 cup (120 ml) neutral oil, and 2 tsp vanilla until smooth.
Bring on the chocolate: Stir 3 oz (85 g) melted dark chocolate into the wet mixture. Make sure it’s warm, not hot, so it blends without scrambling the eggs.
Mix the batter: Pour wet into dry. Whisk until mostly combined, then slowly add 3/4 cup (180 ml) hot brewed coffee.
Batter will be thin—this is good.
Bake: Divide batter evenly between pans. Bake 28–32 minutes, or until a toothpick comes out with just a few moist crumbs. Cool 10 minutes in pans, then invert onto racks.
Peel parchment and cool completely.
Make the fudge frosting: In a heatproof bowl, add 8 oz (225 g) chopped dark chocolate, 1/2 cup (115 g) unsalted butter, and 1/2 cup (120 ml) heavy cream. Set over a pot of barely simmering water and stir until smooth. Off heat, whisk in 1/2 cup (60 g) sifted cocoa and 1 cup (120 g) powdered sugar.
Add 2 tbsp sour cream and 1 tsp vanilla. If too thick, whisk in 1–2 tbsp hot water until glossy.
Level and stack: If domed, trim the cake tops with a serrated knife. Place one layer on a board.
Spread about 3/4 cup frosting over it in an even layer. Top with the second layer.
Crumb coat: Spread a thin layer of frosting over the whole cake to seal crumbs. Chill 15–20 minutes to set.
Final coat: Apply a thicker, even layer of frosting.
Use an offset spatula for smooth sides and soft swirls on top. Aim for a satin finish, not perfection.
Elegant chocolate design: For a simple, polished look: Swooped top: Use the back of a spoon to make concentric curves from center to edge.
Chocolate shards: Melt semisweet chocolate, spread thin on parchment, chill, then break into slender pieces. Press a few into the top at an angle.
Shavings: Use a vegetable peeler along a chocolate bar to make curls.
Scatter on one side for an asymmetrical accent.
Finishing touch: Lightly dust half the surface with cocoa through a fine sieve, then add a pinch of flaky sea salt for sparkle.
Set and serve: Let the cake rest 20–30 minutes so the frosting firms slightly. Slice with a warm, clean knife for neat pieces.