Makes one 15cm/6in layer cake.
Preheat the oven to 180C/350F/gas mark 4. Grease three 15cm/6in round cake tins and line the bases with baking parchment.
Whisk together the cocoa powder and hot coffee until smooth then whisk in the buttermilk, set aside. Sift together the flour, bicarbonate of soda, salt and ground cardamom, set aside.
Place the butter, sugar and vanilla extract in a large bowl and whisk with an electric mixer until smooth and light. Whisk in the eggs one at a time, whisking really well after each addition, then whisk in the melted chocolate.
Divide the batter evenly between the tins (I use a scale to get them exactly even) and level the surface. Bake for about 25 minutes (check at 20) until a skewer inserted into the center comes out clean or with a few moist crumbs.
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)).
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.
Place the chocolate in a heatproof bowl then heat the cream until it is just boiling. Pour the cream over the chocolate, let it sit for a minute then stir until smooth and the chocolate has completely melted. Stir in the butter until melted then set aside to cool slightly, but don't leave it so long that it starts to set.
Pour the ganache over the top of the cake and encourage it to drip over the edges with the back of a spoon. leave it to set slightly then place the remaining buttercream in a piping bag fitted with a star nozzle (or whatever piping nozzle you prefer and pipe eight dollops of buttercream on top of the cake.
Sprinkle over some chocolate sprinkles and serve. Store in an airtight container at room temperature.