Heat milk in a small pan over moderate heat until it comes to a boil.
In blender or food processor combine egg, sugar, a pinch of salt, semisweet chips, and liqueur. Run processor or turn on blender to low setting. Pour in boiling milk in a slow stream. The hot milk will cook egg and melt chocolate. Process or blend 1 minute, until smooth.
Spoon chocolate into 4 demitasse cups and chill. After dinner, beat cream until soft peaks form. Add a little sugar and beat to combine. Top the chocolate cups with a dollop of cream and garnish each cup with mint sprigs.
Place cups on saucers and serve with demitasse spoons. If you use teacups, this recipe will yield 2 cups, rather than 4.