1.Preheat oven to 120°C (100°C fan forced). Grease oven trays, line with baking paper.
2.Beat egg whites and caster sugar in small bowl with electric mixer about 15 minutes or until sugar is dissolved, fold in sifted cocoa.
3.Spoon mixture into piping bag fitted with 2cm (¾ inch) fluted tube, pipe 4cm (1½ inch) stars onto trays 2cm (¾ inch) apart. Bake about 1 hour. Cool on trays.
4.Beat butter in small bowl with electric mixer, beat in sifted icing sugar, milk and extract until combined.
5.Sandwich meringues with peppermint filling, dust with extra sifted cocoa powder before serving, if you like.