This recipe for meringue mushrooms is courtesy of Johnny Iuzzini.
Preheat oven to 250 degrees. Line a baking sheet with parchment paper or a nonstick baking mat; set aside.
Place egg whites, sugar, and lemon juice in the bowl of an electric mixer. Place bowl over a saucepan of simmering water, and whisk constantly while rotating the bowl until sugar is dissolved and mixture is warm. Attach bowl to electric mixer fitted with the whisk attachment and beat until stiff peaks are formed, about 10 minutes.
Fit a pastry bag with a 1/2-inch plain round tip. Transfer meringue to pastry bag. To create mushroom stems, pipe meringue in an upwards motion onto the prepared baking sheet, applying even pressure until you have a strip of meringue about 1 inch in length with a point at one end and a flat side at the other. To create mushroom caps, hold the pastry tip about 1/4 inch above the prepared baking sheet and apply pressure to the bag until a 3/4-inch round is formed. Continue making stems and caps as desired, making sure to reserve about 1/4 cup meringue for attaching the stems to the caps.
Dust mushroom caps with cocoa powder. Transfer baking sheet to the oven and bake until meringue is firm, about 1 hour.
Using a small paring knife, make a tiny incision in the center of the flat underside of the mushroom caps. Pipe a tiny bit of reserved meringue. Place the caps over the pointed ends of the stems, return mushrooms to baking sheet and continue to bake until meringue is hard and completely dry, about 2 hours. Store in a covered container at room temperature.