Place a piece of aluminium foil on a hotplate and warm up a small metal star-shaped cookie cutter shortly afterwards. Cut out stars from half of the chocolate bars, place them on baking paper and refrigerate briefly.
Whisk the egg whites. Sift icing sugar and add to the egg white together with the lemon juice, liqueur and peppermint oil. Stir briefly with the whisk of the hand mixer. Pour the mixture into a piping bag with a star-shaped spout and squirt a tuff onto each of the remaining chocolate bars.
Decorate with the stars immediately and let dry at room temperature. Serve dusted with icing sugar. Makes about 48 pieces.