Set the oven to 200°F (93°C) and prepare two baking sheets by lining them with parchment paper.

In a clean, dry mixing bowl or the bowl of a stand mixer fitted with a whisk attachment, beat the egg whites on medium speed until foamy. Add the salt and cream of tartar, continuing to beat until soft peaks form.

Gradually add the granulated sugar, one tablespoon at a time, while beating continuously. Continue to whip until stiff, glossy peaks form and the sugar has fully dissolved.

Transfer the meringue mixture into a piping bag fitted with a large round tip (or simply cut the tip of the bag). On one baking sheet, pipe small round mounds for the mushroom caps. On the second sheet, pipe tall, narrow dollops for the stems. Smooth any peaks with a damp fingertip if necessary.

Place both trays in the preheated oven and bake for 1 hour and 30 minutes, or until the meringues are crisp and dry. Turn off the oven and allow them to cool completely inside with the door closed to prevent cracking.

Using the tip of a paring knife, gently carve a small cavity into the underside of each cap. Fill the hollow with a small amount of melted chocolate, then insert the pointed end of a stem. Press lightly to secure. Allow the chocolate to set at room temperature.

Once the chocolate has hardened, dust the tops of the meringue mushrooms lightly with cocoa powder if desired. Use to decorate a Yule Log cake or serve as a delicate holiday confection.