In a small bowl, whisk together the peanut butter, soy sauce, sriracha, rice vinegar, and the garlic powder. Add water as needed to thin sauce to desired texture.
Heat sesame oil in a large skillet over medium high heat.
Slice bok choy, in half, lengthwise. Brush each cut half of the bok choy generously with the peanut sauce mixture.
Place bok choy cut side down on the grill or in the skillet, and cook until it is seared, about 2-3 minutes. Flip over each piece, and brush the back side of the bok choy with the peanut sauce mixture.
Place bok choy on a serving dish, and drizzle with remaining peanut sauce mixture. Sprinkle sesame seeds over the top.