Preheat your oven to 200°F. Line a large baking sheet with parchment paper or a silicone baking mat, and set aside.
In a grease free bowl, combine egg whites, salt, and cream of tartar or vinegar/lemon juice. Using a stand mixer (recommended!), electric beaters, or a handheld whisk (don't recommend … ouch!), beat the egg white mix on medium high speed until it turns an opaque white color and just starts to froth.
Gradually add your sugar to the egg white mixture (about 1 tablespoon every 30 seconds) beating well after each addition until all of the sugar is used up.
Continue beating the mixture until it turns thick and glossy and you no longer feel any dry granules of sugar. You can test this by rubbing a bit of batter from the bottom of the bowl between your fingers, and it shouldn't feel grainy. The egg whites should be at a stiff peak as well. If using, whisk your vanilla extract into the meringue mixture.
Transfer the meringue batter to a piping bag fitted with a large plain piping tip. Pipe half of the batter into cookie disks (about 1 inch wide) onto your prepared baking sheets to resemble caps. Sprinkle some cocoa powder on top to resemble dirt. Pipe the remaining half of the batter into taller cookies that are about 1/2 an inch wide and 1 inch tall to resemble stems. For all the cookies, you only need to leave a small space between each cookie for air to circulate. But they can be quite close together, if needed. You should get around 20 caps and 20 stems out of this recipe.
Bake in the center of your preheated 200°F oven for 1 hour, or until the cookies easily slide off the parchment paper and sound hollow if gently taped on the bottom. Turn the oven off, and leave the cookies in the warm oven with the door shut for another hour to dry out. Cool the cookies completely before assembling.
Assemble the cookies: If the tops of your tall cookies (mushroom stems) are uneven, shave a bit off the top using a knife to get a flat surface. Set aside.
Turn the flat cookie disks (mushroom caps) over to reveal the bottom. Spread a thin layer of melted chocolate onto the bottom of the caps (about 1/2 a teaspoon). Flip your "mushroom stems" over and stick the top of the stem to the bottom of the cap. Refrigerate for about 20 minutes, or until the chocolate has hardened.