Preheat oven to 175 degrees Celsius. Grease three 5-inch round tins with butter and line the bottom with parchment paper. Set aside.
Place the softened unsalted butter, caster sugar and brown sugar in a large mixing bowl. Sift in all-purpose flour, cocoa powder, baking soda, baking powder and salt. Add in vanilla extract and milk. Using an electric mixer, mix the mixture on low for 1 minute, then turn the speed up to high and beat for 2 minutes.
Add in the eggs and continue to beat on high for another minute.
Divide the batter among the three cake tins. Bang the tins twice on the table counter each to release excess air bubbles. Bake for 20 to 25 minutes until a skewer inserted into the centre comes out clean.
Let the cake cool completely on the cake tin before frosting
Combine instant espresso powder, sugar and boiling water in a small bowl. Stir until instant espresso powder and sugar has dissolved. Let cool completely before using.
Coffee Swiss Meringue Buttercream
Make sure your butter is soft and diced before you start the recipe - you should be able to press into the butter easily with your finger but it shouldn't look glossy.
Dissolve instant espresso powder into boiling water. Set aside.
Get ready a small pot and a heatproof mixing bowl that can fit snugly on the small pot. Fill the small pot with water until the surface of the water is 1-inch away from the bottom of the mixing bowl. Bring the water to a boil and turn down to a simmer.
Combine egg white and sugar in the heatproof mixing bowl. Place the heatproof bowl over the pot of simmering water and whisk with a wire whisk constantly until the mixture reaches 60C/140F. (If you don't have a thermometer, use fresh egg whites - best is pasteurised - and whisk until you cannot feel any more sugar lumps.)
Remove the mixing bowl from the pot of simmering water and using an electric hand mixer (a stand mixer won't work as the quantity is too little), whisk the mixture on medium speed until it forms a stiff meringue. Continue beating on low speed until the meringue has cooled down - the mixing bowl should be barely warm to the touch.
Turn the mixer speed up to medium again and add in the diced butter tablespoonful by tablespoonful, beating until the butter has been incorporated before adding the next spoonful.
Once all the butter has been added in, keep beating until the mixture comes together into a nice-looking buttercream. Beat in the coffee.
Place dark and white chocolate in separate small bowls.
Place cream in a pot and heat on medium-high heat, stirring frequently, until it comes to a simmer (it should not be boiling).
Pour 40 grams of cream into the dark chocolate bowl and remaining 20 grams of cream into the white chocolate bowl. Cover both bowls with a lid to prevent excess evaporation and set aside for 5 minutes.
Remove the lid and stir with a whisk until the chocolate has melted and the ganache is smooth. If there are still some unmelted chocolate, microwave at 15-seconds intervals, whisking after each interval, until all the chocolate has melted.
Let the ganache cool at room temperature until it has thickened into a pouring consistency (you can speed up the cooling in the refrigerator, if needed).
Unmould the cake layers and slice off the top if needed to ensure a flat layer. Brush the Coffee Syrup onto the cake layers.
Place a 5-inch cake board (trim it to 5-inch if possible) on a cake-decorating turntable. Dab the middle of the cake board with a bit of the Coffee Swiss Meringue Buttercream and place a cake layer on the cake board (to glue the cake onto the cake board).
Spread 1/3 of the Coffee Swiss Meringue Buttercream onto the cake layer. Top with a second cake layer, spread with another 1/3 of the Coffee Swiss Meringue Buttercream and top with the last cake layer. Spread a very thin layer of Coffee Swiss Meringue Buttercream on the sides of the cake and spread the rest on top of the cake. Chill the cake in the refrigerator for 30 minutes for the frosting to set.
If the chocolate ganache has hardened, microwave at 15-seconds intervals, whisking after each interval, until the ganache is smooth and pourable. Transfer the white chocolate ganache into a piping bag.
Remove the cake from the refrigerator. Pour the dark chocolate ganache on top of the cake, spreading it carefully with an offset spatula so that some of the ganache drip down the sides of the cake.
Snip off the tip of the white chocolate ganache piping bag and pipe a spiral starting from the center of the cake. Make a spider web design by running a skewer or toothpick from the center of the cake to the edge of the cake, cleaning the skewer/toothpick after each stroke.
You can chill the cake in the refrigerator until the ganache has set to get clean cuts, however, the cake is best eaten when it's at room temperature.