Preheat the oven to 210°F (100°C). Line a baking sheet with parchment paper.

To make Swiss meringue, place egg whites in the clean bowl of a stand mixer or a large metal bowl. Add powdered (icing) sugar and whip egg whites with a hand whisk a few times. Put the bowl over the bain-marie or water bath and beat egg whites with a whisk until foamy.

When the meringue mixture is hot, remove the bowl from the heat, and beat it with a stand or electric mixer until it cools completely. The meringue will have a glossy appearance with stiff peaks. Use it right away.

Transfer meringue to a large piping bag with a round piping tip 1/2" (12 mm). Pipe 4 meringue dots in the corners of the prepared baking sheet and place parchment paper. It will fix the paper during pipping and baking.

To pipe the meringue stems, hold the pastry bag upright on the baking sheet. Start piping gently, forming a stem's base, then lift the bag while squeezing. Make sure to make the pointed end by pulling up the bag.

For the caps of the mushrooms, keep holding the piping bag upright to the baking sheet and smoothly squeeze the bag. Push the meringue into a plain round shape. Once the perfect shape is obtained, stop squeezing and slowly remove the bag off to the side to avoid creating a peak on top. If needed, use a wet fingertip to flatten the spike of the meringue dome.

The size of mushroom caps and the height of stems may vary: while the caps may average 1 to 2 inch (2.5 cm to 5 cm) rounds and about 1 inch (2.5 cm) high, the stems may get about 1 inch (2.5 cm) tall. Caps and stems should be placed 1 inch (2.5 cm) apart. Sprinkle mushroom caps with cocoa powder. Bake caps and stems for 1 hour. Once the meringue is ready, let it cool and detach from parchment paper.

To assemble meringue mushrooms, carefully select and arrange the caps and stems to achieve the most visually appealing size and shape combinations. Melt chocolate in a double boiler or microwave.

Then, use one of the provided methods for assembly. Don't worry about slightly crooked stems: they will appear perfect when flipped over. However, adjust any completely fallen stems.

Slightly trim the tops of the mushroom stems evenly using a serrated knife. Using a small offset spatula, coat the bottom of the cap with melted chocolate. Dip the tip of a mushroom stem into the chocolate and twist it gently to attach the cap and stem together. Place the assembled mushroom upside down on a wire rack lined with parchment and allow the chocolate to set. 

Another way to assemble the meringue mushrooms is to make a small hole on the bottom of each cap with the tip of a small paring knife. Fill the hole and the top of the stems with a little melted dark chocolate. Insert the stems into the hole under the caps. Place the mushrooms upside down and refrigerate to allow the chocolate to set.

Sprinkle cocoa powder on the upright meringue mushrooms to give them a freshly emerged look from a soft earth patch.