In a medium saucepan, combine the soy sauce, sugar, pineapple juice, garlic, ginger, and black pepper. Bring to a boil over medium heat, stirring occasionally. When the mixture begins to foam/rise, immediately remove from the heat.
Remove 1 cup of the mixture from the saucepan. Set it aside and let it cool to room temperature. This will be used as a marinade.
Place the salmon filets in the marinade and let marinate for 30 minutes to 2 hours—any longer and the salmon may become mushy.
Combine the cornstarch and water in a small bowl and whisk to create a slurry.
Place the saucepan with the remaining sauce over medium-high heat. Slowly whisk the cornstarch slurry into the sauce. Continue stirring until the sauce thickens to your desired consistency, about 1 to 2 minutes. Remove from the heat and let cool completely before transferring to an airtight container to refrigerate. The sauce will keep in the refrigerator for up to 2 weeks.
Preheat the grill to 350°F. Oil the grill grate with vegetable oil to prevent sticking.
Cook the salmon for 4 to 6 minutes per side or until the internal temperature reaches 130 to 145°F. The fish should release easily when cooked thoroughly.
Drizzle some of the thickened sauce over the salmon and serve with rice and stir-fried vegetables.