Preheat oven to 400F. Line a baking sheet with foil and set aside.
In a large bowl, combine tamari, water, honey, ground ginger, and garlic powder and stir to combine.
Pour half of the sauce into a small sauce pan. Leave the other half in the bowl. Add the salmon pieces to the bowl and cover with plastic wrap. Let sit for 20 minutes or so to marinate.
In the meantime, heat the sauce in the sauce pan until it comes to a boil.
In a small bowl, combine the cornstarch and cold water until the cornstarch is no longer lumpy. Pour the cornstarch slurry into the sauce and stir.
Bring the sauce to a simmer and let cook for about 5 minutes until it is thick. Remove from heat and set aside.
Place the salmon on the baking sheet lined in foil. Bake for 12-15 minutes, until salmon is cooked through.
Remove from the oven and sprinkle with sesame seeds, cilantro or green onions.