- 3 large egg whites, at room temperature
- 1/2 cup granulated sugar
- 1 cup confectioners’ sugar
- 1/2 cup Dutch-processed cocoa powder
Preheat the oven to 275 degrees. Line a rimmed baking sheet with parchment paper. Place the egg whites in the bowl of a stand mixer fitted with the whip and beat on medium speed until foamy. Begin adding the granulated sugar a tablespoon at a time, adding it until the egg whites have increased in volume. Raise the speed to medium-high, add all of the remaining granulated sugar, and beat for about 5 minutes, or until stiff peaks form. Lower the speed to medium and continue to beat for an additional 2 minutes to incorporate more air into the whites. They should be very light and stuff, but not dry and separated.
Remove the bowl from the mixer and, using a rubber spatula, fold the confectioners sugar into the egg whiles, taking care not to deflate them and making sure you reach all the way to the bottom of the bowl.
Scrape the meringue into a pastry bag fitted with a 1/4-inch plain top. To make the mushroom stems, hold the pastry bag at a 90-degree angle and squeeze dime-sized portions of meringue onto the prepared cookie sheet. As you finish squeezing each stem, quickly pull the bag straight up to that you leave a little tail of meringue. Make as many stems as you will need, keeping in mind that baked meringue is fragile and breaks easily, so make a few more that you will actually need. (I like to make enough mushrooms so that everyone can have one when the cake is served).
Make an equal number of mushroom caps by holding the pastry bag at a slight angle with the tip touching the parchment paper as you begin to pipe individual meringues into 1/2-inch rounds. When you have a 1/2-inch round, immediately stop piping and pull the tip straight up, leaving no tail.
Place the cocoa powder in a fine-mesh sieve and, tapping on the side of the sieve, lightly dust the mushroom caps with cocoa.
Place the meringues in the oven and bake, with the door slightly ajar to allow the moisture to escape, for about 1 hour, or until very dry and firm. If the meringues take on color, the oven is too hot. Remove the pan from the oven, lower the heat, and let the oven cool slightly before returning the pan to continue baking.
Turn off the oven and allow the meringues to cool in the oven for about 1 hour. This helps dry them completely. If you don’t have time to do this, remove the pan from the oven and carefully transfer the meringue pieces to a wire rack to cool.
Store the mushroom pieces in an airtight and very dry container until ready to put them together when you assemble the cake. (If making the mushrooms for another use, you will need some tempered chocolate or buttercream to glue the caps to the stem as directed in the recipe).