For the cake
6 large eggs, separated into yolks and whites, at room temperature
¼ cup plus 2 tablespoons sugar
1 teaspoon pure vanilla extract
4 ounces bittersweet chocolate, chopped and melted
¾ teaspoon cream of tartar
2 tablespoons cake flour
For the mousse
4 large egg whites, at room temperature
⅛ teaspoon salt
¼ cup sugar
1½ cups chilled heavy whipping cream
8 ounces bittersweet chocolate, chopped, melted and kept lukewarm
½ cup mini chocolate chips or chopped chocolate (optional)
For the ganache
10 ounces bittersweet or dark chocolate chopped
1¾ cups heavy cream
About 1 cup chocolate sprinkles (optional)
For the cake: Preheat the oven to 350 degrees. Lightly grease a rimmed baking sheet with cooking oil spray, then line the bottom with parchment paper.
Combine the egg yolks and ¼ cup of the sugar in the bowl of a stand mixer or use a handheld electric mixer; beat on medium-high speed, until light, thick and fluffy, for about 5 minutes. The mixture should nearly triple in volume and fall off the beaters in a thick ribbon. Add the vanilla extract; beating on medium-high speed, then add all the melted/cooled bittersweet chocolate, and beat until just incorporated.
Beat the egg whites in a separate, clean bowl using the balloon-whisk attachment on your stand mixer or with the handheld electric mixer on medium-high speed; once they are foamy, add the cream of tartar. Increase the speed to medium-high and beat until soft peaks form. Gradually add the remaining 2 tablespoons of sugar, beating until stiff peaks form.
Gently fold a small amount of beaten egg whites into the egg yolk-chocolate mixture, to loosen it up. Fold in a third of the remaining beaten egg whites (some white streaks can remain), then sift in a tablespoon of the cake flour and gently fold to incorporate. Repeat with the remaining beaten egg whites and the remaining tablespoon of cake flour until everything is just incorporated.
Transfer the batter to the baking sheet, trying not to deflate it. Use an offset spatula to spread it evenly into the corners. Bake (middle rack) for 17 minutes, until the top of the cake springs back when lightly pressed. The cake should be a light brown, similar in color to brown sugar. Transfer the pan to a wire rack to cool completely.
For the mousse: Combine the egg whites and salt in the bowl of a stand mixer fitted with a balloon-whisk attachment or use a handheld electric mixer; beat on medium-high speed until soft peaks form. Gradually add the sugar, beating until glossy, soft peaks form.
Beat the heavy whipping cream in a separate clean bowl (using your stand mixer fitted with a balloon-whisk attachment or a handheld electric mixer) until soft peaks form. By hand, fold half the lukewarm chocolate into the whipped cream with a spatula until incorporated, then fold in the remaining chocolate to form a smooth, homogenous mixture.
Gently fold one-third of the beaten egg whites into the chocolate whipped cream until just blended, then fold in the remaining beaten egg whites. Gently fold in the mini chocolate chips or chopped chocolate, if using. Cover and refrigerate the mousse until chilled, but no longer than 30 minutes, or the mousse may become too stiff to work with.
When you are ready to assemble the cake, make the ganache: Place the 10 ounces of bittersweet or dark chocolate in a heatproof bowl. Heat the heavy cream in a medium saucepan over medium-high heat, just until bubbles form at the edges. Immediately pour over the chocolate; let the mixture sit for a minute or two, then whisk to form a smooth, glossy ganache. Let it cool for a bit while you begin the cake assembly.
Invert the cake slab onto a cutting board and discard its parchment paper. Cut the cake into 3 rectangles of equal size. Place one portion on a large serving platter or plate. Use a piping bag or offset spatula to spread half the chilled mousse on the bottom cake layer.
Next, stack another portion of cake on the mousse layer, pressing it lightly in place without forcing out any of the mousse below. Follow with the remaining mousse, then top with the remaining cake portion, again pressing it lightly in place.
Use an offset spatula to spread the slightly cooled ganache over the top and sides of the cake, keeping things neat. If desired, press handfuls of the chocolate sprinkles against the sides of the cake (or decorate as you wish). Refrigerate for at least 30 minutes, and up to overnight.
Let the cake come to room temperature for 15 to 20 minutes before serving.
A guide to grilling basics, from prep