Place the chicken breast between 2 pieces of plastic wrap and hammer flat until it's larger than the bun
Season both sides of the chicken with salt and pepper
Heat oil in a grill pan until sizzling.
Brush one side of the chicken liberally with some of the piri piri sauce. Lay it in the grill pan sauce side down and grill until the bottom of the chicken whitens, 5-6 minutes. Don't move the chicken too often to check, or you'll ruin the grill marks- once the sides are white.
Flip the chicken, brush with more sauce and continue cooking another 1-2 minutes
Add the cheese to the top of the chicken and cover til fully melted. Remove the chicken from the pan and allow to rest.
Butter the buns and toast them on the grill pan til golden brown- 1-2 minutes.
spoon some aioli on the bottom bun, add the lettuce, then tomato, then chicken and top with more sauce, then the top bun.