Wash the cauliflower. Tir it tightly in a cheesecloth bag and cook until tender in boiling salted water. Drain and put in a baking dish.
Melt the butter, stir in the flour and blend well. Stir in the milk slowly, and when the sauce bagins to thicken, stir in the egg yolk, sugar, cheese and salt. Cook and stir a few minutes longer to let the sauce get quite thick, then remove from the fire. Let it rest a few minutes.
Crush the rusks.
Pour the sauce over the cauliflower and sprinkle the top with the crushed rusk crumbs.
Place in a preheated 400° oven 15 minutes to brown.