Photograph: Dan Jones
Cook the Brussels sprouts in a large pan of boiling salted water until just tender. Drain and keep warm in a serving bowl.
Melt the butter in a pan, add the breadcrumbs and chestnuts and cook, stirring, until the crumbs are golden. Add the parsley; season and stir. Scatter the mixture over the sprouts. Serve.
Get ahead: prepare the sprouts (and make the breadcrumbs, if necessary) the night before.