Preheat broiler. Mix together peanut butter, water, sesame oil, soy sauce, sambal oelek, and rice vinegar. Toss chicken thighs and Broccolini in sauce, and place on a rimmed baking sheet. Broil 10 inches from heat until chicken is cooked through, about 10 minutes (watch to make sure it doesn't burn).
I just made this. I used broccoli instead of broccolini, and doubled the sauce because I doubled the chicken. The sauce is ok, but it needs something, like fish sauce. If I make this again, I would thin out the sauce a little more, add a couple of teaspoons of fish sauce and maybe some sugar. Good idea, definitely needs some tweaking.