In a large mixing bowl, combine the dark chocolate, semi-sweet chocolate, cocoa powder, cubed butter, instant espresso powder, vanilla extract, and salt. Set this mixture aside for now. In a medium saucepan over medium heat, add the heavy cream and granulated sugar. Stir continuously until the sugar is completely dissolved and the mixture reaches a gentle simmer. Carefully pour the hot cream mixture over the chocolate and butter ingredients. Whisk everything together until the chocolate and butter are fully melted and the mixture becomes silky, smooth, and glossy. At this stage, the frosting will be quite thin. Place the bowl in the refrigerator and allow it to chill for 3 to 4 hours, or until the frosting thickens to a spreadable consistency. It usually firms up perfectly in the time it takes to bake and cool the cakes, so you can begin the cake preparation right after.

Preheat your oven to 350°F (175°C). Grease two 8-inch round cake pans, line the bottoms with parchment paper, and dust the sides lightly with flour. Tap out any excess flour and set the pans aside. In a large bowl, whisk together the all-purpose flour, cocoa powder, granulated sugar, brown sugar, baking soda, baking powder, and salt until well combined. Add the vegetable oil, sour cream, eggs, and vanilla extract to the dry ingredients, and whisk until the batter is smooth and free of streaks. It will be quite thick at this point. In a separate cup, dissolve the instant espresso powder into 1 cup of boiling water. Pour this hot espresso mixture into the cake batter and whisk until everything is well incorporated and smooth. Divide the batter evenly between the two prepared pans. Bake on the center rack of your preheated oven for 35 to 40 minutes, or until a skewer inserted into the center comes out with a few moist crumbs attached. Allow the cakes to cool in their pans for about 10 minutes. Then place a piece of parchment paper on top of each cake and invert them onto a wire cooling rack—this helps prevent sticking, as the cakes are very moist. Let the cakes cool completely to room temperature before assembling.

In a small bowl, whisk together the boiling water, instant espresso powder, cocoa powder, and granulated sugar until fully dissolved. Allow the mixture to cool for 5 to 10 minutes at room temperature before using.

Make sure the fudge frosting has thickened enough to spread before beginning assembly. If it’s still too soft, continue checking it in the fridge every 30 minutes until it firms up. Once the cake layers are completely cooled, use a serrated knife or cake leveler to even out the tops. Then, carefully slice each cake in half horizontally to create four even layers. To assemble, place the first layer on a serving plate or cake board. Brush the top with a generous amount of the coffee soak, then spread a thin, even layer of frosting over it. Repeat this process with the remaining layers—brushing with soak, then frosting, until all four layers are stacked. Apply a thin layer of frosting over the entire cake to create a crumb coat that seals in loose crumbs. Chill the cake in the refrigerator for 20 to 30 minutes to allow the crumb coat to set. Once chilled, frost the outside of the cake with the remaining frosting until smooth and evenly coated. If desired, decorate with chocolate curls or shavings. Keep the cake refrigerated until ready to serve.