Preheat oven to 210°F (100°C) and line a baking sheet with parchment paper.
In a mixer fitted with the whisk attachment, whip the egg whites and cream of tartar on low speed until foamy.
Slowly add in the sugar and mix on high speed until stiff peaks.
Fill a piping bag fitted with a large plain tip with the meringue.
Pipe the stems: with the piping tip held directly over the baking sheet begin piping while moving up and away from the sheet.
Pipe the caps: keep the piping tip low and close to the baking sheet and pipe the round cap shape. If necessary smooth out the top with a slightly wet finger. Lightly dust the caps with cocoa powder.
Bake for 45-50 minutes. Turn the oven off and allow the meringue to cool inside.
Once completely cooled, melt the chocolate using a microwave or a double boiler.
Using a knife, carve out a small hole in the centre of each cap. To assemble, dip a cap and the tip of a stem in chocolate. Place the stem in the carved out hole of the cap and let it harden. Repeat with the remaining stems and caps until all mushrooms are assembled.