Add the stout to a medium saucepan and place over medium high heat. Bring the stout to a light boil before reducing the heat to a simmer. Keep an eye on the stout in the beginning as it can begin to foam over if it boils too vigorously. Use a ladle to skim off any excess foam. As the stout begins to reduce, the foam should subside. Continue simmering until the stout has been reduced to 1 cup, about 15 to 20 minutes.
Once the stout has reduced, add the demerara sugar and stir until dissolved. Remove from the heat and cool before using. The syrup will keep for 2 to 3 weeks in the refrigerator.
Stout Old Fashioned
Prepare a short rocks glass by adding ice and set aside. Add several ice cubes to a cocktail shaker and top with the scotch whisky, the stout demerara syrup, and the bitters. Use a cocktail spoon to stir the mixture for 5 to 10 seconds. Strain over the ice in the rocks glass.
Add the stout to a tall glass. Use a milk frother to create a thick foam. If you don't have a milk frother you can add the stout to a small bowl and use a whisk to create a foam. The foam won't be as thick as with the milk frother but it should still work. Spoon a layer of the stout foam on top of the cocktail.
Use a rasp to grate fresh nutmeg over the cocktail and enjoy!
Notes
*Brown sugar can be substituted for the demerara sugar