Combine egg yolk , coffee and sugar in a saucepan ; cook over low heat , stirring constantly with a wooden spoon until the mixture leaves a thin coating on your spoon. DO NOT LET IT BOIL.
Place saucepan in a bowl of ice water and stir until mixture is barely warm.
Transfer the mixture to a bowl and Add the cocoa ; With a electric mixer on high speed , beat until thick enough for mixture to form a ribbon when it falls from the beaters. Beat in the butter a little at a time until all mixed in.
Whip heavy cream until stiff and fold into the mixture.