Mix 6 tablespoons of milk and the cornstarch. Heat the remaining milk in a pot. Mix the cornstarch mixture, sugar, salt, cocoa powder and vanilla seeds. Remove the milk from the heat and slowly stir in the cocoa mixture. Bring back up to a boil, stirring constantly. Remove from the heat and let cool slighlty.
Rinse a jelly mold with cold water. Mix the egg yolk with 1 tablespoon of the pudding. Stir rapidly into the hot, not boiling, pudding. Pour the pudding into the jelly mold. Chill for at least 4 hours. Beat the cream and sugar until stiff. Fillet the clementine. Briefly dip the mold in cold water and plunge the pudding onto a serving plate. Serve garnished with cream and clementine fillets.