In a small saucepan, combine soy sauce, brown sugar, mirin, water, garlic, ginger, and sesame oil. Simmer over medium heat until sugar dissolves.
Mix cornstarch with 2 tablespoons of cold water to make a slurry. Whisk into the sauce and simmer until thickened, about 1-2 minutes. Set aside.
Cut salmon into 1 1/2-inch cubes, pat dry with paper towels.
Preheat Blackstone to medium-high heat. Oil the surface lightly.
Place salmon pieces on the griddle, leaving space between each piece. Cook for 2-3 minutes until the bottom starts to caramelize.
Flip once and brush with teriyaki sauce. Cook another 2-3 minutes until salmon is just cooked through.
Transfer to a serving plate, brush with additional sauce, and garnish with sliced green onions and sesame seeds. Enjoy!