Reduce the heat to medium and simmer uncovered for 10 to 12 minutes, or until the liquid is reduced by half.
In a small bowl, stir together the cornstarch and water to make a slurry; stir it into the sauce and simmer for another minute, or until thickened.
1.5 teaspoons cornstarch
Let the black bean sauce cool, then store in an airtight container (preferably glass).
Notes
Keeps for up to 2 weeks in the fridge.
For extra flavor (depending on how you plan to use it), add about 0.5 teaspoon of orange zest. Add it with the broth, then continue simmering.