Place room-temperature egg whites in the bowl of an electric mixer fitted with a whisk attachment. (Note: It is important that both the bowl and whisk are very clean so that the egg whites whip properly.) Beat the egg whites on medium speed.
Once they are very frothy, stop the mixer and add the cream of tartar. Start the mixer again and continue to beat the egg whites. Once they form soft peaks, increase the speed to high and gradually add the sugar, one tablespoon at a time.
Continue beating the whites until they are very shiny and hold stiff peaks, but are not dry or crumbly. Spoon the meringue into a large pastry bag fitted with a 1/2-inch round tip.
First, pipe the mushroom caps. Hold the pastry bag at a 90 angle, about 1/2 inch from the parchment paper. Using firm and even pressure, squeeze out a round meringue disc about two inches in diameter and one inch high. Stop squeezing, then twist the bag and lift it from the meringue to get a clean break from the cap. Repeat in regular intervals on the baking sheet until you have approximately two dozen mushroom caps.
The mushroom caps will most likely have peaks on top, so smooth the tops by wetting your index finger and lightly pressing the peaks down into the caps.
Next, pipe the mushroom stems. Again, position the bag perpendicularly, about 1/2-inch from the baking sheet. Begin squeezing the bag to form a one inch round base. Continue to squeeze as you slowly and evenly draw the bag up, then release pressure (causing a break) to form a tapering stem about two inches tall. The stems will have peaks on top, but do not smooth them down. Use the remaining meringue to pipe as many stems as possible.
Bake the meringues at 200 F for 90 minutes, turning them halfway through the cooking time to ensure an even bake. The meringues should be hard and dry to the touch, and you should be able to easily lift one from the parchment. Allow them to cool completely before assembling. If time allows, you can also turn off the oven and let them sit in the oven for several hours or overnight.
Use a small, dry artist's brush, apply the red dry food color to the baked meringue caps. Apply a second coat to intensify the color.
To assemble the mushrooms, melt the chocolate at 30-second intervals in the microwave. Use a toothpick to carve a small hole in the bottom of the mushroom cap.
Use a toothpick to add a dot of chocolate to the hole.
Place the pointed end of a mushroom stem into the hole in the red cap.
As they are assembled, place the mushrooms on parchment paper and leave them to set. Assemble the remaining caps and stems into mushrooms.
Put matcha powder in separate small bowls. Dip the bottoms of the mushroom stems in the matcha powder.
Place the honey and white nonpareils in separate condiment cups. Use an artists’ brush to place small dots of honey onto the red mushroom caps.
Use an artist's brush to place small dots of fresh meringue onto the red mushroom caps.