In a small bowl, combine the brown sugar, water, tamari, and sriracha, and stir to dissolve the sugar. Reserve ⅓ of the mixture. Pour the rest into a pie plate or similar dish, add the salmon fillets, and turn to coat. Let sit at room temperature while the oven preheats, about 10 minutes.

Preheat the oven to 425°F. Line a baking sheet with foil.

Turn the salmon again in the marinade, then place on the prepared baking sheet, skin-side down. Discard the marinade. Bake for 4-6 minutes per ½ inch of thickness of the salmon fillets; 4 minutes per ½ inch will give you a moister center, while 6 minutes per ½ inch will give a flakier consistency.

While the salmon is baking, place the reserved marinade in a small skillet or saucepan and bring to a boil over high heat. Cook for 1-2 minutes, or until the sauce has reduced in volume by half and has become syrupy.

Place the cooked salmon fillets on a serving dish and spoon the reduced sauce over the top. Serve immediately.