Using an electric mixer, whisk eggwhites until soft peaks form then, with motor running, add caster sugar in a thin steady stream and continue whisking until stiff peaks form and mixture is thick and glossy. Using a metal spoon, fold in icing sugar, malt powder and cocoa, then transfer mixture to a piping bag fitted with a 1cm-diameter plain tube.
Preheat oven to 100C. Pipe thin 8cm diameter rounds of meringue mixture onto baking paper-lined oven tray and bake for 2 hours or until crisp but not coloured, turn off oven and cool completely.
Melt chocolate in a heatproof bowl over a saucepan of simmering water, dip edges of meringues in chocolate, shake to remove excess, then place on baking paper-lined oven trays and refrigerate. Meringues will keep for 3-4 days stored between layers of baking paper in an airtight container in the refrigerator.