Beat the yolks with the sugar until they turn to foam. Add the coffee, pour this into a pot and place it on the heat, in order to thicken, while stirring continuously, and then add the bulk of the cream.
The mousse is ready once it begins to separate from the bottom of the pot. Cool and stir with the remaining whipped cream. Distribute it into bowls and sprinkle it with walnuts if desired.
Put it in a refrigerator to solidify.