Melt the dark chocolate, either in a heatproof bowl over a pan of simmering water, or in short bursts in the microwave. Set aside to cool slightly while you make the meringue.
Place the egg whites and sugar in a large, spotlessly clean heatproof bowl (the bowl of your stand mixer if you have one) and place the bowl over a pan of gently simmering water; do not let the base of the bowl touch the water. Gently whisk the mixture until the sugar has dissolved, if you rub some between your fingers you shouldn't feel any graininess. It should reach at least 60C/140F on a thermometer (but don't let it get too hot or it will start to set (at 70C)).
Remove the bowl from the heat and whisk on a high speed with an electric mixer until it has formed a thick, glossy meringue and is completely cool to the touch, about 10 minutes.
Turn the speed down to low and add the butter a tablespoon at a time, whisking well after each addition, until you have added all the butter and the buttercream has become smooth and silky. It may become soupy or curdled looking, just keep whisking and it will come back together. (If it doesn't, you may need to add a couple of tablespoons more of butter. Or if it remains soupy, refrigerate it for 15-30 minutes then carry on whisking on a high speed. If it remains curdled, place a couple of heaped tablespoons of the mixture in a separate bowl and microwave it until it melts, then drizzle it back into the rest of the buttercream slowly, while whisking on a high speed.)
Once the buttercream is smooth, whisk in the vanilla extract, espresso powder, cardamom and salt on a low speed, then fold in the dark chocolate (which by now should have cooled, but still be melted). Place the buttercream in a piping bag fitted with a large, plain nozzle.
Place one of the cooled cakes on a serving plate or cake stand and pipe rings of the buttercream to completely cover the top of it (you can just spread it on if you prefer.
Place the second cake layer on top and repeat the piping. Place the final layer on upside down so that the top is completely flat. Press down gently then place the cake in the fridge while you make the ganache.