Place the cornstarch in a small saucepan and whisk in 2 tablespoons of the soy sauce to create a slurry.
Whisk in the remaining soy sauce, mirin, garlic, ginger, granulated sugar, and brown sugar.
Turn the heat up to medium. Continue whisking frequently until the sauce thickens, approximately 5 minutes. Whisk in the sesame oil and remove from the heat.
This marinade can easily be doubled or more, though it may slightly increase the time required to reduce the sauce. For reference, this recipe yields currently enough sauce for approximately 4 salmon filets or chicken breasts.