Lightly butter a 30cm diameter, 7.5cm deep, round springform tin and line the base with baking parchment.
Set a large heatproof bowl over a saucepan of very gently simmering water, making sure the base of the bowl does not touch the water. Dissolve the coffee in
Whisk the egg whites with the salt to stiff peaks. Using a large spoon or spatula, fold a big spoonful of egg whites into the chocolate mixture. Once combined, fold in the rest. Tip into the tin, smooth the top and
Remove from the oven, leave in the tin for about 5 minutes, then turn out on to a wire rack to cool completely.
For the ganache, melt the chocolate, cream and butter in a bowl set over very gently simmering water, again ensuring the base of the bowl does not touch the water. Stir until smooth, then cool a little to thicken before pouring over the cake. Allow to set for at least 2 hours.