Preheat oven to 425°. Trim leaves and sides of stalks from broccolini. Put broccolini on 2 large rimmed baking sheets (they will be very full).
Melt butter with salt and pepper in a small saucepan over medium heat. Stir in chile powder, remove from heat, and stir in lime zest. Drizzle butter over broccolini and turn until well coated.
Roast broccolini, switching pan positions and turning with tongs halfway through cooking, until tender-crisp, 15 to 18 minutes.
Make ahead: Up to 1 day, chilled. Reheat, tented with foil, in a 350° oven on 1 large rimmed baking sheet until hot, 30 minutes.
Note: Nutritional analysis is per serving.