In a mixing bowl, combine the dark chocolate, semi-sweet chocolate, cocoa powder, and salt. In a small saucepan, heat the heavy cream and granulated sugar over medium heat until sugar is dissolved, about 3-5 minutes. Once melted, pour the hot cream mixture over the chocolate mixture and whisk until smooth. Refrigerate for at least 1 hour.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, dark brown sugar, baking powder, baking soda, and salt. In another bowl, blend the eggs, hot coffee, and melted dark chocolate until smooth. Gradually add the wet mixture to the dry ingredients, stirring gently until just combined.

Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans. Divide the batter evenly between the prepared pans. Bake for about 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Once the cakes are out of the pans, let them cool completely on the wire rack for at least 1 hour.

In a bowl, whisk together the instant coffee and boiling water until dissolved. Allow it to cool slightly.

Once cooled, level the tops of the cakes if necessary, and slice each layer in half horizontally. Place one layer on a serving plate, brush generously with the coffee soak, and add a layer of frosting on top.

Apply a thin crumb coat around the sides and on top of the layered cake. Chill in the refrigerator for about 20 minutes.

Once the crumb coat has set, coat the top and sides of the cake with remaining frosting to create a smooth finish.