If you want spotted mushrooms, use a toothpick or a small, new paintbrush to “paint” spots of melted chocolate on the mushroom caps. Try to make these cookies on a cool, dry day, as humidity or rain tends to make meringue soft or cause it to bead.
Gently squeeze out caps and stems onto a paper-lined cookie sheet using a pastry bag filled with the egg white-sugar mixture. To create nicely rounded caps, hold the piping tip close to the cookie sheet.
Spread 1/2 teaspoon of melted chocolate on the underside of each meringue cap. Insert the pointed end of a stem into each cap, pressing slightly to secure. Dry upside down on a rack until the chocolate sets.