Soak gelatin in a bowl of cold water until softened, about 5 minutes. In a pot, heat milk with espresso powder and salt. In a bowl, whisk egg yolks with sugar and vanilla sugar. Place bowl over a pan of hot water and gradually whisk in hot milk. Squeeze gelatin and stir into egg mixture until dissolved. Stir in coffee liqueur. Place the bowl over another bowl filled with cold water and stir until mixture is cold. Chill briefly in the refrigerator.
Whip the cream. When coffee gelatin begins to solidify, fold in whipped cream. Divide cream among glasses and garnish with chocolate mints. Chill until ready to serve. Garnish with lemon balm.