Preheat oven to 200 degrees. Lightly grease 3 cookie sheets and line them with parchment paper.
Beat egg whites until foamy. Add lemon juice and beat at medium speed until soft peaks form. Continue beating, gradually adding superfine sugar until whites are very stiff and glossy. If meringue batter feels gritty with sugar, continue beating until all sugar has melted.
Fill a pastry bag with ¼ cup of meringue and fit with 1/16-inch plain nozzle. Set aside to use for gluing mushroom cap and stem together. Put remaining meringue in a large pastry bag fitted with a plain nozzle.
To pipe caps, hold pastry bag upright almost touching the parchment paper and pipe meringue into even rounds, building up meringue to form a 2-inch wide round, about 1 inch high. (You can make varying sizes.) Try to make the top as smooth as possible; use a wet fingertip to smooth out any peaks.
To pipe stems, hold pastry bag upright almost touching parchment-lined cookie sheet and pipe meringue with even pressure, lifting the bag slowly away from cookie sheet to form a cone-shape stem base, a little larger than the top. Stem should be about 1 inch high. Try to keep stems as straight as possible. (Some stems may fall on their sides while baking, so it’s a good idea to make extra.)
Bake meringues for approximately one hour or until mushrooms are firm enough to be lifted from baking sheet without sticking. Rotate baking sheets from top to bottom and front to back in oven (about three quarters of the way through baking) to ensure even baking.
Remove from oven and with a small sharp knife, make a small hole in the middle of the underside of each mushroom cap. Using the small pastry bag fitted with 1/l6 inch tip, pipe a bit of meringue in the hole and gently press the top of stem into the hole.
Place mushrooms, caps down, on a parchment-lined baking sheet and return to oven for about 15 to 30 minutes or until mushrooms are dry.
Remove from oven and lightly dust tops of the mushrooms with cocoa powder. Use a small pastry or paint brush to smudge the cocoa powder, if desired.
Notes
If you don’t have superfine sugar, process granulated white sugar in a food processor.