Step 1

In a small saucepan, combine the soy sauce, sesame oil, garlic, ginger, brown sugar, and red pepper flakes if using

Step 2

Bring to a simmer over medium heat

Step 3

Meanwhile, in a small bowl, combine the cornstarch with ½ cup cold water

Step 4

Stir the cornstarch mixture into the sauce and return it to a simmer

Step 5

Cook until the sauce has thickened, about 5 minutes

Step 6

Store refrigerated in an airtight jar container for up to 1 week