With a needle, make a hole about 2mm in diameter in the pointed end of the eggs and a second slightly bigger hole (about 3mm in diameter) at the other end. Blow through the smaller hole to drain the contents of the egg. Gently wash the emptied eggshells and dry on a rack.
Heat 500ml of cream in a saucepan over very low heat. Once hot remove from the heat and stir in small pieces of white chocolate. Stir constantly until smooth. Set aside. Repeat in another pan with the dark chocolate.
With a pastry bag with a fine tip fill three egg shells with the dark chocolate and 3 others with white chocolate.
Refrigerate for at least 8 hours to harden the chocolate. Serve as is. The surprise is guaranteed.