Concord Cake

In a heatproof medium bowl set over a pot of simmering water, melt the chocolate with the butter, stirring occasionally, until smooth. Remove from the heat and keep warm.

In a small saucepan, combine the sugar and water and boilover moderately high heat, without stirring, until the sugar syrup reaches 240° on a candy thermometer, about 7 minutes. 

Meanwhile, in the bowl of a stand mixer fitted with the whisk, beat the egg whites at medium-high speed until soft peaks form, about 3 minutes. 

Gradually pour the hot syrup into the egg whites in a steady stream and beat at medium-high speed until the whites are stiff and glossy, about 5 minutes. Using a large rubber spatula, fold in the melted chocolate until no streaks of white remain. Scrape the chocolate mixture into a large bowl. Wash and dry the mixing bowl and whisk. 

In the stand mixer fitted with the whisk, scrape the vanilla bean seeds into the heavy cream. Beat the cream at medium speed until firm. Using a rubber spatula, fold the whipped cream into the chocolate mixture until no streaks remain. Cover the mousse with plastic wrap and refrigerate until firm, at least 2 hours or overnight.

Meanwhile, in a medium bowl, sift the 2 cups of confectioners’ sugar with the cocoa powder. Line 4 rimmed baking sheets with parchment paper. Trace an 8-inch circle on 2 of the sheets. 

In the stand mixer fitted with the whisk, beat the egg whites at low speed until foamy. Increase the speed to medium-high and beat until soft peaks form. Beat in the granulated sugar 3 tablespoons at a time, beating well after each addition. Once all of the sugar has been added, beat the whites until stiff and glossy, about 3 minutes longer. 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. 

Preheat the oven to 350°. Bake the meringue rounds for about 25 minutes, until they are firm and can be lifted off the parchment with an offset spatula. Transfer to racks to cool completely. 

Bake the meringue sticks for 12 to 14 minutes, until firm enough to be lifted off the parchment. Cut them into 6-inch lengths and transfer to a rack to cool completely. Using a sharp knife, cut the sticks into 1 1/2- to 2-inch lengths. 

Transfer a meringue round to a cake stand or platter. 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 on top, 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.