For meringue: Preheat oven to 225°F (105°C). Using a dark marker, trace 2 (8-inch) circles on a sheet of parchment paper, and place, ink side down, on a baking sheet. Trace another 8-inch circle on another sheet of parchment paper, and place, ink side down, on another baking sheet.
In a medium bowl, whisk together confectioners’ sugar and cocoa.
In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites at medium speed until soft peaks form, 3 to 4 minutes. With mixer on medium speed, add granulated sugar in a slow, steady stream; increase mixer speed to medium-high, and beat until sti peaks form and sugar fully dissolves, 4 to 5 minutes. Sift confectioners’ sugar mixture onto egg mixture in three additions, folding just until combined after each addition. Spoon meringue into a pastry bag fitted with a ½-inch round piping tip (Ateco #806).
Starting from center of each drawn circle, pipe meringue in a spiral to fill each circle. Pipe remaining meringue in long strips in remaining space on prepared pans. (Try to keep pressure on the pastry bag as even as possible and let the meringue fall from the bag as opposed to pressing it into the parchment to keep its tubular shape. Make sure the strips are not too thin or too thick, as this will result in an uneven bake.)
Bake until completely dry and set and meringue strips easily pull away from parchment, about 1 hour. Turn oven off, and let meringues cool in oven for 1 hour. Cooled meringues can be stored in airtight containers for up to 2 days.
For mousse: In the top of a double boiler, heat chocolate over simmering water, whisking frequently, until melted. Remove from heat, and whisk in room temperature cream.
In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites and salt at medium speed until soft peaks form. With mixer on medium speed, gradually add granulated sugar; increase mixer speed to medium-high, and beat until stiff peaks form, 3 to 4 minutes. Transfer to another bowl.
In the bowl of a stand mixer fitted with the whisk attachment (bowl and whisk do not need to be cleaned between), beat cold cream at mediumspeed until stiff peaks form. Add whipped cream to chocolate mixture, folding until combined. Add egg white mixture, folding until combined and no streaks remain. Use immediately.
Remove cooled meringue strips from parchment paper. Using a sharp paring knife, cut meringue strips into 2-inch pieces.
Using a large offset spatula, place 1 meringue disk on desired serving platter. Spread 1½ cups (about 270 grams) mousse on top of meringue. Top with second meringue disk, and spread 1½ cups (about 270 grams) mousse on top. Top with remaining meringue disk. Spread remaining mousse on top and sides of cake. Arrange meringue strip pieces on top and sides of cake as desired. Refrigerate for at least 4 hours or up to overnight before serving. (The long chill helps to soften the meringue layers and make them easier to slice.) Cut with a serrated knife. Refrigerate in an airtight container for up to 2 days, but best served within 1 day.
