Preheat the oven to 135 °C and line a baking tray with parchment paper.
Whip the egg whites and cream of tartar first on low speed, then once the whites are foamy increase the speed and slowly pour in the sugar while whipping. Continue to whip until the whites on high speed until they hold a firm peak when beaters are lifted.
Spoon the meringue into a piping bag fitted with a large plain tip. Pipe the stems of the meringue by piping upwards while piping out meringue (pipe 2-3 extra, in case they tip when baking). Pipe 12 caps by keeping the piping tip low to the tray to create the mushroom cap shape. If the caps have a point to them from lifting the piping bag, tap them down with a finger dipped in icing sugar.
Bake the meringues for about 45 minutes, until they are dry once cooled - they can be returned to the oven to dry further, even after cooling. The outdoor temperature and humidity can affect the baking time (heat and humidity making the meringues take longer to bake).
Melt the chocolate in a metal bowl place over barely simmering water, stirring gently until melted.
To assemble, dust the tops of the mushroom caps lightly with cocoa powder. Use a skewer to poke a small hole in the bottom of each cap. Use a small brush to pain chocolate on the underside of each mushroom cap and insert the pointed stem end and set back on the baking tray until the chocolate has set.
The mushrooms can be made up to 2 days in advance and stored in an airtight container.