Combine dark chocolate and semi-sweet chocolate in a bowl. Heat heavy cream with sugar until dissolved, pour over chocolate, add instant coffee and whisk until smooth. Chill for 30 minutes.

Preheat the oven to 350°F (175°C) and grease two 9-inch round cake pans. Sift flour, cocoa powder, baking powder, baking soda, and salt in a bowl. Whisk together sugars, eggs, oil, vanilla extract, and hot coffee in another bowl. Combine the wet and dry ingredients until smooth.

Divide batter evenly between pans and bake for 25-30 minutes. Check with a toothpick; it should come out clean. Allow to cool in pans for 10 minutes before transferring to a wire rack.

Mix instant coffee granules with boiling water in a small bowl until dissolved. Let cool slightly.

Slice each cooled layer in half horizontally for four layers. Brush coffee soak on each layer, spread frosting between layers and stack.

Apply a thin layer of frosting over the entire cake and refrigerate for 15-20 minutes.

Smooth the remaining frosting over the cake and decorate as desired.