Makes 3 1/2 cups (6 to 8 servings)
8 ounces bittersweet chocolate , chopped fine
2 tablespoons cocoa powder (preferably Dutch-processed)
1 teaspoon instant espresso powder
5 tablespoons water
1 tablespoon brandy
2 large eggs , separated
1 tablespoon sugar
1/8 teaspoon table salt
1 cup heavy cream , plus 2 additional tablespoons (chilled)
1. Melt chocolate, cocoa powder, espresso powder, water, and brandy in medium heatproof bowl set over saucepan filled with 1 inch of barely simmering water, stirring frequently until smooth. Remove from heat.
2. Whisk egg yolks, 1 1/2 teaspoons sugar, and salt in medium bowl until mixture lightens in color and thickens slightly, about 30 seconds. Pour melted chocolate into egg mixture and whisk until combined. Let cool until just warmer than room temperature, 3 to 5 minutes.
3. In clean bowl of standing mixer fitted with whisk attachment, beat egg whites at medium-low speed until frothy, 1 to 2 minutes. Add remaining 1 1/2 teaspoons sugar, increase mixer speed to medium-high, and beat until soft peaks form when whisk is lifted, about 1 minute. Detach whisk and bowl from mixer and whisk last few strokes by hand, making sure to scrape any unbeaten whites from bottom of bowl. Using whisk, stir about one-quarter of beaten egg whites into chocolate mixture to lighten it; gently fold in remaining egg whites with rubber spatula until a few white streaks remain.
4. In now-empty bowl, whip heavy cream at medium speed until it begins to thicken, about 30 seconds. Increase speed to high and whip until soft peaks form when whisk is lifted, about 15 seconds more. Using rubber spatula, fold whipped cream into mousse until no white streaks remain. Spoon into 6 to 8 individual serving dishes or goblets. Cover with plastic wrap and refrigerate until set and firm, at least 2 hours. (The mousse may be covered and refrigerated for up to 24 hours.)