Chocolate on chocolate on chocolate. What else would one expect from a cake named for a swarthy gypsy violinist, the man who stole the wife of a Belgian prince and scandalized Parisian society for years? Ladies — to your fainting couches! This tale of passion, love and loss may be too much for your sensibilities to bear.
For the spongecake:
2.5 ounces (1/2 cup) all-purpose flour
3 tablespoons cocoa powder
4 ounces (1 stick) butter
6 ounces bittersweet chocolate, chopped fine
10 eggs, room temperature and separated
4.5 ounces (about 2/3 cup) sugar
1/4 teaspoon salt
For the chocolate glaze:
10 ounces bittersweet chocolate, chopped
1.5 ounces butter
1.5 ounces (3 tablespoons) light corn syrup
1 teaspoon vanilla extract
For the filling:
1 ounce (2 tablespoons) cocoa powder
2 ounces (1/2 cup) powdered sugar
8 ounces semisweet chocolate, chopped fine
1 pint heavy cream
1 ounce (2 tablespoons) rum
1 teaspoon vanilla extract
For the assembly:
apricot glaze (if desired)
Preheat your oven to 350. Line a half (13″ x 18″) sheet pan with parchment paper and brush it with melted butter.
Now prepare the cake. Whisk together the cocoa powder and flour. Melt the butter in the microwave in a medium bowl. Add the chocolate pieces and stir to melt, apply ten-second bursts of heat as necessary. When the mixture is smooth, set it aside to cool.
In a mixer fitted with a whip, beat the egg yolks on high until pale and thick, about five minutes, then with the machine running add half the sugar in a stream. Continue whipping until the yolks and sugar fall from the whip in a ribbon. Transfer the yolk mixture to a medium bowl and fold in chocolate mixture. Set the bowl aside while you prepare the whites.
Rinse and dry the mixer bowl and whip. Put the whites in the bowl and whip to soft peaks, add the salt, then add the remaining sugar in a stream. Continue to whip to the stiff peak stage, the foam should be very glossy. Stir 1/3 of the whites into the yolk mixture to lighten the batter, then fold in the rest in two additions. Lastly, fold in the flour and cocoa.
Scrape the batter into the prepared pan and level it with an icing spatula. Bake for 12-15 minutes. Remove the cake from the oven and loosen the edges with a knife. Lay a second sheet of parchment and a wire rack over the layer, then invert the pan. Remove the pan and — carefully — the top sheet of parchment. Cover the cake with a clean kitchen towel and another wire rack and invert the cake again. Remove the parchment from the top and allow the cake to cool.
When the cake is cool, cut the sheet in half. Prepare the glaze by combining all the ingredients save for the vanilla in a microwave-safe bowl. Heat the bowl for 20 seconds on full power, then apply as many ten-second bursts as needed to melt the chocolate completely, stirring in between to allow the residual heat to do most of the work. Stir in the vanilla and pour the glaze over the cake, spreading it promptly with an icing spatula.
Allow the glaze to set, about 45 minutes to an hour. When it’s firm, score the top to delineate twenty 2″ square pieces. Then heat the knife under tap water, dry it and gently make the cuts (you’ll need to clean your knife and re-heat it after each cut).Set the pieces aside or store in the refrigerator.
For the filling, melt the chocolate pieces in the microwave, using a 20-second burst, then as many ten-second bursts as needed until it’s completely smooth. Cool it until it’s just barely warm to the touch. Stir together the cocoa and powdered sugar. Whip the cream mixture to just shy of soft peaks. Then, with the machine on, whip in the cocoa and sugar mixture. Add the vanilla and rum and whip for a few more seconds to incorporate. Add the melted chocolate all at once and quickly whip or whisk it in, whichever works better for you.
Set the second cake layer on sheet of parchment paper and trim up the sides. Brush apricot glaze over it, then apply the filling, being careful to apply it evenly. Square up the corners. Gently lay the glazed top pieces on the top, then refrigerate the entire cake for at least an hour. When ready to serve, cut the cake into pieces using a sharp knife heated under hot top water.