Combine and boil. Combine all ingredients for the BBQ sauce, except the cornstarch, in a medium saucepan and bring to a boil, whisking regularly.
1 cup low sodium soy sauce, 3/4 cup water, 1/2 cup dark brown sugar, 1/4 cup mirin, 1/4 cup rice wine vinegar, 2 Tablespoons gochujang paste, 1 Tablespoon sesame oil, 4 cloves fresh garlic, 2 teaspoons fresh ginger
Simmer. Reduce the heat to low and simmer for 10 minutes. Turn the heat off.
Make thickener. In a small bowl, whisk the cornstarch and cold water until no clumps remain.
2 Tablespoons cornstartch, 1/4 cup cold water
Add thickener. Slowly pour the slurry into the BBQ sauce while whisking constantly.
Combine Mixes. Turn the heat on the saucepan to high and bring the sauce back to a boil while whisking. It should become thicker and more clear as the cornstarch cooks. Add additional water as needed, one Tablespoon at a time, to reach desired consistency.
Cool it. Remove the sauce from the heat and allow to cool before using. Store the sauce in a sealed jar in your refrigerator and use within 2 weeks.