Step 1

1 For the cake: Preheat the oven to 350 degrees

Step 2

2 Lightly grease a rimmed baking sheet (half sheet pan size, 17 by 12 inches) with cooking oil spray, then line the bottom with parchment paper

Step 3

3 Combine the egg yolks and 1/4 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

Step 4

4 The mixture should nearly triple in volume and fall off the beaters in a thick ribbon

Step 5

5 Add the vanilla extract; beating on medium-high speed, then add all the melted/cooled bittersweet chocolate, and beat until just incorporated

Step 6

6 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

Step 7

7 Increase the speed to medium-high and beat until soft peaks form

Step 8

8 Gradually add the remaining 2 tablespoons of sugar, beating until stiff peaks form

Step 9

9 Gently fold a small amount of beaten egg whites into the egg yolk-chocolate mixture, to loosen it up

Step 10

10 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

Step 11

11 Repeat with the remaining beaten egg whites and the remaining tablespoon of cake flour until everything is just incorporated

Step 12

12 Transfer the batter to the baking sheet, trying not to deflate it

Step 13

13 Use an offset spatula to spread it evenly into the corners

Step 14

14 Bake (middle rack) for 17 minutes, until the top of the cake springs back when lightly pressed

Step 15

15 The cake should be a light brown, similar in color to brown sugar

Step 16

16 Transfer the pan to a wire rack to cool completely

Step 17

17 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

Step 18

18 Gradually add the sugar, beating until glossy, soft peaks form

Step 19

19 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

Step 20

20 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

Step 21

21 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

Step 22

22 Gently fold in the mini chocolate chips or chopped chocolate, if using

Step 23

23 Cover and refrigerate the mousse until chilled, but no longer than 30 minutes, or the mousse may become too stiff to work with

Step 24

24 When you are ready to assemble the cake, make the ganache: Place the 10 ounces of bittersweet or dark chocolate in a heatproof bowl

Step 25

25 Heat the heavy cream in a medium saucepan over medium-high heat, just until bubbles form at the edges

Step 26

26 Immediately pour over the chocolate; let the mixture sit for a minute or two, then whisk to form a smooth, glossy ganache

Step 27

27 Let it cool for a bit while you begin the cake assembly

Step 28

28 Invert the cake slab onto a cutting board and discard its parchment paper

Step 29

29 Cut the cake into 3 rectangles of equal size

Step 30

30 Place one portion on a large serving platter or plate

Step 31

31 Use a piping bag or offset spatula to spread half the chilled mousse on the bottom cake layer

Step 32

32 Next, stack another portion of cake on the mousse layer, pressing it lightly in place without forcing out any of the mousse below

Step 33

33 Follow with the remaining mousse, then top with the remaining cake portion, again pressing it lightly in place

Step 34

34 Use an offset spatula to spread the slightly cooled ganache over the top and sides of the cake, keeping things neat

Step 35

35 If desired, press handfuls of the chocolate sprinkles against the sides of the cake (or decorate as you wish)

Step 36

36 Refrigerate for at least 30 minutes, and up to overnight

Step 37

37 Let the cake come to room temperature for 15 to 20 minutes before serving