line the base of a 8-9 inch shallow springform cake pan with silicone baking parchment paper, buttering the pan lightly to hold it in place.
melt the chopped chocolate in a heatproof bowl set over a pan of simmering water. as soon as it starts to soften, add the coffee and leave it for two or three minutes. stir very gently, then when the chocolate has melted add the butter. stir until it has melted.
meanwhile, beat the egg whites with an electric mixer till stiff, then fold in the sugar. mix the baking powder with the cocoa powder and flour. remove the chocolate from the heat, quickly stir in the egg yolks, then slowly, firmly, and gently fold the melted chocolate into the egg whites. lastly sift in the flour and cocoa mixture.