Heat the oven to 225F. Line two cookie sheets with parchment paper and set aside.
Place the egg whites and salt into the bowl of a stand mixer, and with the whisk attachment, mix on high speed until foamy.
Slowly add the sugar and whip until stiff, glossy peaks form.
Add the vanilla extract and beat to combine.
Transfer the meringue to a large pastry bag fitted with a 1/2" tip (Wilton 1A). Pipe mushroom tops, about 1/2" to 1 1/2" diameter, leaving about an inch apart between them.
Put a little bit of water into a small bowl. Using a clean finger, dip your finger in the water, wiping off the excess, and gently tap down the peaked tops of the mushroom tops.
Pipe the mushroom stems by holding the piping bag directly above the parchment, about 1/2" or so from the parchment. Apply pressure to pipe a thicker base with a tapered, thinner top in a variety of sizes. It's best to pipe these as straight up and down as possible to keep them from falling over when baking.
Using a fine sieve, sift the natural cocoa powder on top on the mushroom tops. Sift as little or as much cocoa powder as you would like.
Bake the meringue mushrooms for about 60-75 minutes until the meringue is dry to the touch, and can be easily removed from the parchment. I start checking the meringues every so often about 50 minutes into baking. Let the meringues cool completely.
Fill a small piping bag with melted chocolate. Cut off the tip of the piping bag. You are going to need just a small amount of chocolate per mushroom.
Using a small paring knife, make a small hole in center of flat side of each mushroom top. Pipe a little bit of chocolate into the hole and fit the smaller end of meringue stem into hole. Let the chocolate harden before decorating with the mushrooms.