In a small bowl, sprinkle the gelatin over the water and let stand until softened.
Meanwhile, in a medium bowl, using a handheld electric mixer, beat the cream until softly whipped. Refrigerate until chilled, about 10 minutes.
In a small, microwave-safe bowl, heat the coffee liqueur at high power until hot, about 45 seconds. Stir in the softened gelatin until dissolved.
In another medium bowl, beat the egg yolks with the sugar and salt at high speed until pale and thickened, about 5 minutes. While beating the yolks, beat in the coffee-gelatin liquid; scrape the side and bottom of the bowl. Beat in the melted chocolate. Using a rubber spatula, fold in the whipped cream in 2 additions. Scrape the mousse into a bowl, cover with plastic wrap and refrigerate until firm, at least 4 hours or overnight.
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