Using a double boiler, melt the chocolate.
Transfer to a saucepan. Use a rubber spatula to make sure you get all of the chocolate.
Add 1 cup water, cinnamon, salt and sugar.
Bring to a boil. Reduce to a simmer and cook for 5 minutes.
Add milk and bring to the boil again.
Reduce to a simmer.
Make a slurry by whisking together the potato starch with 4 tablespoons water in a small bowl.
Add to the porridge and whisk well until thoroughly incorporated.
Simmer 5 minutes.
Serve warm or chilled.