Preheat oven to 200 degrees F. Line baking sheet(s) with parchment, set aside.
In small sauce pan add the egg whites, cream of tartar and sugar (this can also be done over a double boiler). Over medium heat Whisk until the sugar is dissolved (you can do a pinch test and rub fingers together, you should not feel any sugar granuals, if you do keep whisking).
Pour mixture into bowl of mixer fitted with whisk attachment. Whip until mixture is cool and glossy, stiff meringue peaks form.
Mix in the vanilla
If adding cocoa (for a light brown color) or food coloring do so now.
Place meringue in piping bag fitted with a large round tip (I used Ateco #806)
Pipe out our mushroom caps. If peaks form dip your finger into cold water and gently tap down. Sprinkle with sprinkles if using.
Pipe out a stem to go with each mushroom cap.
Bake for 1 hour. Then lower the oven (DO NOT OPEN OVEN DOOR) to 175 degrees F. Continue to bake about 45-60 minutes until dry to the touch and easily comes off parchment paper, be careful that they do not brown.
DECORATING AND ASSEMBLING:
With pointy sharp knife make a hole on the bottom in the center of each mushroom cap.
Melt chocolate and white chocolate. With dark chocolate paint the underside of each mushroom and then paint white chocolate stripes to resemble gills. Let set.
On the stems cut off the pointy tip. Put melted chocolate in the hole of the mushroom cap and instert the stem. Let set upside down until completely dry.
Store in airtight container until ready to serve.