Make a double boiler by placing a large heatproof bowl over a medium saucepan of simmering water on the stove. Add the chocolate and butter to the bowl and stir until completely melted and smooth. Set aside.

Boil the sugar and water in a small saucepan, without stirring, until the sugar syrup reaches 240° Farenheit on a candy thermometer, about 7 minutes. (this clip-on instant-read candy thermometer is ideal for this task).

While the sugar boils, in the stand of a mixer fitted with the whip attachment, beat the egg whites at medium high speed until soft peaks form, about 3 minutes. Pause until the syrup is ready.

With the mixer running at medium high speed, gradually pour in the hot syrup in a steady stream and continue beating at medium-high speed until stiff peaks form, about 5 minutes. Pouring the hot syrup into the egg whites cooks them, so if you are squeamish about raw egg whites, have no fear!

Fold the melted chocolate into the egg whites using a large rubber spatula until barely incorporated, but no white streaks remain. Scrape the chocolate mixture into a separate large metal bowl (you will be adding the whipped cream to it shortly, so make sure you have enough room) and wash and dry your mixing bowl.

Whip heavy cream and vanilla extract in the stand mixer fitted with a whip attachment at medium speed until firm. Fold the whipped cream into the chocolate mixture using a rubber spatula in three batches until no white streaks remain. Cover with plastic wrap, or transfer to an airtight container. Refrigerate until firm, at least 2 hours.

Heat oven to 350 F.

Sift the 2 cups of confectioners’ sugar with the cocoa powder into a medium bowl.

Line 4 rimmed baking sheets with parchment paper. Use the bottom of an 8-inch cake pan pan to trace an 8-inch circle on 2 of the sheets.

Beat the egg whites in the bowl of a stand mixer fitted with the whisk attachment at low speed until foamy. Increase the speed to medium-high and beat until soft peaks form. Add in the granulated sugar 3 tablespoons at a time, beating well after each addition.

Beat whites until stiff and glossy, approximately three additional minutes, once all of the sugar has been added.

Transfer the meringue to a large bowl and, using a large rubber spatula, gradually fold in the cocoa powder and confectioners’ sugar until just a few streaks remain.

Scrape the meringue into a piping bag fitted with a 1/2-inch tip.

Pipe the meringue into the traced circles in a spiral, beginning at the center; there should be no space between the spirals. On the other 2 prepared sheets, pipe the remaining meringue in long sticks, leaving about 1 inch between them. Let the meringue rounds and sticks stand at room temperature for 30 minutes.

Bake the meringue rounds for about 25 minutes, until they are firm and can be lifted off the parchment with an offset spatula. Bake the meringue sticks for 12-14 minutes.

If possible, turn the oven off, and leave in oven overnight to cool thoroughly and dry out.

Cut sticks into 1 1/2- to 2-inch lengths.

Assemble the Cake

Transfer a meringue round to a cake stand or serving plate.

Spoon half of the chilled chocolate mousse onto the meringue and spread it in an even layer with a large offset spatula.

Cover with the second meringue round and spread the remaining mousse to form the top layer, mounding it slightly in the center.

Cover the cake entirely with the meringue sticks. Refrigerate until the mousse is firm, at least 6 hours or overnight. Dust the cake with confectioners’ sugar just before serving.