Here the fat in the eggs and the cream balances the bitterness of the chocolate and coffee, burying their sharp edges and making the mousse round and harmonious.
In a heatproof mixing bowl placed over (not in) a pan of simmering water, melt the chocolate with the coffee, stirring frequently. Remove the bowl from the heat.
Combine the eggs and sugar in a separate heatproof bowl and put it over the simmering water. Whisk constantly until the mixture is light yellow and forms thick ribbons when you lift the whisk. Remove the bowl from the heat, add the melted chocolate, and stir to combine. Cool the mixture to room temperature.
In a clean mixing bowl, whisk the cream to soft peaks. Gently fold half into the chocolate mixture until it is thoroughly blended. Fold in the rest, then transfer the mousse to individual serving bowls (or one large bowl) and chill it in the refrigerator for at least 4 hours. Serve cold.
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