Pat the chicken dry with paper towels.
Season both sides with salt, pepper, and garlic powder.
Let the chicken rest for 10 minutes while preparing other ingredients.
Preheat the grill to medium-high heat.
Grill the chicken with the lid closed until it reaches 165°F internally (about 15–25 minutes depending on cut).
Remove the chicken from the grill and let it rest for 5 minutes.
Slice thinly or shred the chicken.
Toss the chicken lightly with BBQ sauce.
Slice slider buns in half.
Brush the tops with melted butter.
Toast buns on the grill or under a broiler for 2–3 minutes until golden.
Spread Japanese mayonnaise on the bottom buns if using.
Layer BBQ chicken evenly over the buns.
Add sliced onions or onion rings.
Sprinkle cheese and cilantro if desired.
Place the top bun on and gently press to secure. Serve warm.