In a small bowl, mix the soy sauce, rice vinegar, sesame oil, honey, garlic, ginger, and hot chili oil and set aside. In another small bowl, dissolve the cornstarch in 1 teaspoon water and set aside.
Bring the mirin to a boil in a small saucepan over high heat. Reduce the heat to medium-low and simmer for 5 minutes. The mirin will become slightly syrupy.
Then add the soy sauce mixture to the mirin and simmer for another 5 minutes, stirring occasionally. In the final minute or two, add the cornstarch mixture to thicken.
Cool slightly and store, covered, in the refrigerator.