In a medium saucepan, whisk together the soy sauce, 1 cup of water, ginger, garlic powder, brown sugar, and honey. Place over medium heat.
In a small bowl, combine the remaining 1/4 cup water and the cornstarch.
Pour the cornstarch slurry into the saucepan and heat until thickened, whisking occasionally.
Place the chicken breasts into a baking dish in a single layer. Pour the teriyaki sauce over the chicken and let it marinate for at least 30 minutes.
Preheat a grill pan or your grill to medium heat. Grill the chicken breasts until cooked through (internal temperature of 165 degrees F), about 2-3 minutes per side.
Remove the chicken from the grill and allow to rest on a plate covered loosely with foil for 10 minutes.
While the chicken rests, grill the pineapple slices on both sides until golden and heated through.
Spread 1 tablespoon of mayo on the cut side of each top and bottom bun. Top the bottom bun with lettuce, a chicken breast, 2 slices of bacon, and a grilled pineapple slice. Place the top bun onto the sandwich and serve immediately.
Notes
If you can't find onion buns, you can use regular burger buns. The sweet onion flavor really adds something to this sandwich though.