Toss Brussels sprouts with oil, then arrange, cut sides down, in a 17- by 12-inch shallow baking pan. Roast, without turning, until outer leaves are tender and very dark brown, 40 to 45 minutes.
Add butter and toss to coat.
Stir together all dressing ingredients until sugar has dissolved.
Cook cereal, oil, and shichimi togarashi in a small skillet over medium heat, shaking skillet and stirring, until rice is coated and begins to turn golden, about 3 minutes.
Transfer to a bowl and cool, stirring occasionally.
Put Brussels sprouts in a serving bowl, then toss with just enough dressing to coat.
Sprinkle with puffed rice and serve remaining dressing on the side.
·Puffed rice can be made 3 days ahead and kept in an airtight container at room temperature. ·Dressing, without mint and cilantro, can be made 1 day ahead and chilled, covered. Bring to room temperature and add herbs before using.·Brussels sprouts can be roasted 4 hours ahead. Chill, uncovered, until cool, then cover. Reheat, uncovered, in a 350°F oven until hot, 10 to 15 minutes.
