In a medium bowl, whisk together soy sauce, brown sugar, and mirin until sugar dissolves.
Add sesame oil, garlic, ginger, and optional pineapple juice. Mix well.
Place chicken, beef, salmon, tofu, or vegetables in a resealable bag or shallow dish.
Pour marinade over, making sure everything is coated evenly.
Refrigerate and marinate for at least 30 minutes. For deeper flavor, marinate 2–4 hours (fish: 30–60 minutes max).
Cook as desired—grill, pan-sear, or bake. For a glaze, simmer leftover marinade for a few minutes until slightly thickened.