Make the custard: In a medium saucepan, heat 3/4 cup cream over medium-low heat until steaming. In a medium bowl, whisk together the egg yolks, sugar, salt, and espresso until well combined, then pour warm cream into the egg mixture in a slow, steady stream while whisking constantly until well combined. Pour the contents of the bowl back into the saucepan and cook over low heat, stirring constantly, until the mixture has thickened slightly and can coat the back of a spoon. (This only takes a few minutes!)

Pour the custard through a fine mesh sieve into a fresh bowl and stir in the liqueur.

Melt the dark chocolate in a double boiler over medium low heat. (You may also melt the chocolate in a microwave in 20 second intervals, stirring between each.) Remove the chocolate from the heat, whisk in the custard, and let cool.

While the chocolate mixture cools, pour the 1 ¼ cups chilled cream into the bowl of an electric mixer with the whisk attachment. Beat until stiff peaks are just formed. Whisk ¼ of the whipped cream into the chocolate mixture, then gently fold in the remaining whipped cream until thoroughly combined.

Chill, covered, at least 6 hours. Let stand at room temperature 20 mins before serving. (For dirt cups, after folding in the whipped cream, immediately spoon into a piping bag and pipe into clear glasses. Let the mousse set in the refrigerator once the cups have been assembled.)