There are so many recipes for chocolate mousse but Julia Child's, which uses egg yolks and butter, yields silky, smooth and decadent results every time.
Beat the egg yolks and sugar together until mixture becomes a thick, pale yellow, and falls back upon itself forming a slowly dissolving ribbon. Beat in the orange liqueur.2.
Set your mixing bowl over not-quite-simmering water on the stove and continue beating for 3 to 4 minutes until the mixture is foamy and too hot to touch with your finger. Then beat the mixture over cold water for 3 to 4 minutes, until the mixture is cool and forms a ribbon. It will have the consistency of mayonnaise.3.
Melt chocolate with coffee over hot water. Remove from heat and beat in the butter a bit at a time, to make a smooth cream. Beat the chocolate into the egg yolks and sugar. Beat in the orange peel at this step, if desired.4.
In a separate bowl, beat the egg whites and salt until soft peaks are formed; sprinkle on the sugar and beat until stiff peaks are formed. Stir one fourth of the egg whites into the chocolate mixture. Slowly fold in the rest.5.
Gently scoop mixture into a large serving dish, dessert cups, or petits pots. Refrigerate for at least 2 hours or overnight before serving.6.
To serve, add a dollop of whipped cream and enjoy!
Reprinted with permission from Julia Child's Mastering the Art of French Cooking (1961)