Melt the chocolate in a double boiler. Meanwhile, whip the cream with the cream stiffener until stiff. Add the hot chocolate to the cream and carefully fold in with a whisk. Put the mousse in the fridge for about 1 1/2 hours.
Peel the bananas, cut them into fine slices and mix them with a little lemon juice and powdered sugar (this will prevent the bananas from turning brown).
Decorate a plate with powdered sugar and eggnog. Shape mousse au chocolat into dumplings with the help of two spoons and serve on the plate with some of the banana salad.