Put the cauliflowers into a large saucepan of boiling water with 2 teaspoons of salt. The water should just cover them. Boil, covered, for 20 minutes. If not tender continue to boil for another 5 minutes and test again with a skewer. When tender lift the cauliflowers carefully to avoid breaking and put to drain in a colander.
Meanwhile, put the brussels sprouts in about 2½ cups (6 dl) of boiling salted water and boil for 15 minutes or until just tender. Lift and drain very well.
While the vegetables are cooking bring the sauce to just below boiling point, melt the butter and stir it in. Whisk in the egg yolk and then the cream, adding a little salt and pepper. On no account let it boil. Stand it on the side of the stove while arranging the potato in a border around the edge of a warmed, large, flat dish. Put the brussels sprouts in groups of five at intervals on the potato border. Place the cauliflowers in the middle. Pour the sauce over the cauliflowers and serve immediately.