Chop the chocolate into bits and melt with the butter in a bowl over simmering water.
Whip the yolks and
Whip the egg whites, salt, and remaining granulated sugar until it peaks and then fold into the chocolate mixture.
Pour the batter into the cake pan. Bake until center of the cake is moist but not runny, 30 to 40 minutes.
Let the cake cool and turn out onto a platter. Put the confectioner’s sugar in a sieve and dust the top of the cake. Serve the slices of cake with the coffee custard underneath and the Chantilly cream over.
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