In a small bowl, sprinkle gelatin over 2 tablespoons of water. Let sit until gelatin is hydrated.
In a medium bowl, whisk together egg yolks, sugar, and salt until pale in color.
In a small saucepan, heat 1 cup of cream and espresso powder over medium heat until warm. Stir occasionally to make sure cream doesn’t burn. Just before cream starts to boil, gradually whisk cream mixture into egg mixture. Return mixture back into the saucepan and cook over medium heat, constantly stirring. Add 3 tablespoons of whiskey. Cook until mixture thickens.
Transfer to a clean bowl and whisk in hydrated gelatin mixture until incorporated. Press plastic wrap onto the surface of the mixture. Let cool completely at room temperature for 30-45 minutes. Make sure the entire surface of the mixture is covered with plastic to prevent an unwanted skin from forming. Do not refrigerate or freeze mixture!
In the bowl of a stand mixer, whisk 1 cup cream until soft peaks form. You can also whisk the cream by hand. With a spatula, gently fold soft cream into the cooled espresso mixture. Divide mousse among six 4-ounce containers. Cover and chill until set, about 1 hour.
Before serving mousse, whisk remaining 1/2 cup cream and 1 tablespoon confectioners’ sugar until soft peaks form. Garnish chilled mousse with a dollop of whipped cream. Sprinkle with cocoa powder and cocoa nibs.