- 4 egg whites, at room temperature
- 1/2 tsp cream of tartar
- 1 cup granulated sugar
- 1/4 cup cocoa powder
Prepare 2 baking sheets by lining them with parchment paper. Preheat the oven to 200 degrees.
Place room temperature egg whites in the bowl of an electric mixer fitted with a whisk attachment. It is important that the bowl and whisk both be very clean, so that the egg whites whip properly.
Begin beating 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, a tablespoon at a time. Beat 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, or the open end of a coupler.
First, pipe the mushroom caps: hold the pastry bag at a 90-degree angle about 1/2 inch from the parchment paper. Using firm and even pressure, squeeze out a round meringue disc about 2 inches in diameter and 1 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 2 dozen mushroom caps. You can smooth out the tops of your mushrooms by wetting your index finger and lightly running it along the caps.
Next, pipe the mushroom stems. Again position the bag perpendicular about 1/2-inch from the baking sheet. Begin squeezing the bag to form a 1-inch round base. Continue to squeeze as you slowly and evenly draw the bag up, forming a tapering stem about 1.5 inches tall. Use the remaining meringue to pipe as many stems as possible—some stems invariably tilt and collapse, so you should always make extras.
Bake the meringues at 200 degrees for about 90 minutes, turning them halfway through the cooking time to ensure even cooking. The meringues should be hard and dry to the touch, and you should be able to easily lift one from the parchment. Once the meringues are done, turn off the oven and let them sit in the oven for several hours or overnight.
To assemble the mushrooms, melt the white chocolate in a small bowl. Use a toothpick to carve a small hole in the bottom of a mushroom cap. Dip the top of a stem in the white chocolate, and stick the chocolate-covered stem top in the hole of the mushroom cap. Place the mushroom on a baking tray to set, and repeat with remaining caps and stems.
Place the cocoa powder in a sifter, and lightly sift cocoa over the tops of the mushrooms. Mushrooms can be stored for up to a 5-6 days in an airtight container in a cool, dry room. Note that humidity can make the mushroom soft and collapse, so do not place them on a cake or in a refrigerated environment until immediately before serving.