Combine the soy sauce and brown sugar in a mixing bowl together. Stir to combine.
Add in the garlic and onion powders. Stir well to combine. Add salt and pepper to taste.
The marinate will be a pasty consistency and thick like brownie mix only grainier.
I like to bake the salmon in a cookie sheet covered in parchment paper. Makes for super easy cleanup. Evenly coat each salmon fillet with the marinate. A little goes a long way.
Bake at 350 degrees for 15-20 minutes. A little longer if you want. These cook pretty fast.