Make a marinade by blending the vinegar, thai chilis, bell pepper, garlic, sugar, and rosemary, if using.
Season your chicken thighs with salt and pepper to taste (I omitted this step, to be honest – it didn’t seem to need it). Marinate your chicken thighs with half of the sauce for a minimum of 15 minutes and up to two hours.
Preheat your oven to broil (500°F) with the rack set to the middle position. At the same time, cover a baking sheet with aluminum foil and place a rack on top.
Meanwhile in a small saucepan, bring the other half of the sauce up to a very low simmer and hold it there until you are done.
When you are done marinating, position the chicken on the rack spacing each thigh about ½” apart.
Place the chicken in the oven, check back every 5 minutes or so for signs of burning, you want a little char, but not too much.
After 15 minutes has elapsed, flip the chicken and place back in the oven for another 15 minutes.
Remove and serve with the simmered piri piri sauce.
