Line a loaf tin with baking paper. Allow the ice cream to soften slightly, then divide it among three bowls or containers, and place two of the three in the freezer. Heat the liqueur, if you are using it, in the microwave until warm. Stir the cocoa powder into the warm liqueur until smooth. Stir the cocoa mixture into the bowl of ice cream that is not in the freezer, combining well. Spoon the flavoured ice cream into the prepared loaf tin to form the first layer, then place in the freezer until firm.
Remove one of the two bowls from the freezer and allow to soften slightly. Finely chop half of the fresh mint, and add it to the ice cream along with a few drops of green food colouring.
Mix well, then spoon on top of the first layer. Return to the freezer and freeze until firm. Remove the remaining bowl of ice cream from the freezer and allow to soften slightly. Stir in the crushed Peppermint Crisp.
Spoon on top of the second layer. Cover with clingwrap and freeze overnight if possible, or until firm. To serve melt the white chocolate and allow to cool
slightly. Invert the loaf tin on a serving platter and place a warm cloth over it to release the ice cream. Peel off the lining paper. Place the melted chocolate in a small plastic bag, cut off a corner and drizzle the chocolate over the ice cream.
Decorate with the remaining fresh mint leaves.