Combine everything except the liqueur and ginger in a small saucepan and place over medium-low heat. Whisk occasionally until chocolate is melted, about 2 minutes.
Continue to cook, whisking frequently until smooth and slightly thickened, about 5 minutes. Remove from heat, stir in liqueur, top with a piece of ginger, if using, and serve immediately.
Recipe courtesy of Aida Mollenkamp