Combine Ingredients: In a medium saucepan, mix together soy sauce, water, sesame oil, sugar, minced garlic, and grated ginger.
Heat Mixture: Place the saucepan over medium heat and bring the mixture to a gentle simmer, stirring frequently to dissolve the sugar.
Thicken Sauce: Add the cornstarch mixture to the saucepan, whisking continuously to prevent any lumps. Let it simmer for 3 to 4 minutes, or just until the sauce begins to thicken.
Add Heat: If using, stir in the red pepper flakes for an extra kick.
Cooling and Storage: Take the sauce off the heat and give it time to cool down. Transfer the sauce to a glass jar or bottle and refrigerate until needed.