Boil the egg for 12 minutes in a pan of water, then drain and run under cold water until cooled. Peel the egg and cut carefully around the hard yolk. Discard the whites.
Blanch the salad cress for at least 15 seconds in boiling water and allow to cool. Put the salad cress, mayonnaise and some seasoning in a food processor. Pulse until blended. Spoon the mayonnaise into a bowl.
Crumble the egg yolk and sprinkle over the top of the mayonnaise. Blanch the watercress for at least 15 seconds in boiling water and allow to cool. Then garnish with some cress and serve with the rice cakes, or as a starter, with carrot batons for dipping, if you like.