Have your cake ingredients ready at room temperature. Preheat oven to 350°F. Lightly spray a 10” x 15” jellyroll pan with cooking spray, line with parchment paper, and wrap it up the sides of the pan, lightly spray paper.
To make the cake batter use a clean, grease-free metal bowl and beater, whisk the egg whites until soft peaks form, add in the lemon juice, and gradually add in 3tablespoons of sugar, one tablespoon at a time. Set aside.
In another bowl sift together the flour, ½ cup sugar, baking powder, and salt. In another bowl, whisk the egg yolks, water, oil, and vanilla. Slowly add the egg yolk mixture to the flour mixture, whisking until well blended. Gently fold in ½ of the egg white mixture into the batter, and then the remaining egg whites. Pour into prepared pan using an offset spatula, ensure the batter is evenly spread right to the corners. Bake for 13-15 minutes, or until the cake bounces back when touched and the toothpick in the center comes out clean. Remove cake from oven; allow to cool in pan.
To build the cake, thoroughly line an 8” wide by 4 to 5” deep (freezer safe) bowl with plastic wrap leaving excess hanging over sides. Remove the cake from the baking pan, measure halfway on the long edge, and cut the cake in half. Then cut a 7.5” circle from one corner, set aside, this will be your base. Using the other half (8 x 10”), make notches at 2” intervals on the 8” sides, cut the cake across the width, creating 10” long, thin triangles, you will be left 8 triangles. With one triangle at a time, carefully line the inside of the bowl starting in the middle, keep the short 2” edge near the top of the bowl and the point in the bottom. Alternate the direction of the triangles so the short 2” edge of each triangle line up around the top of the bowl. Cut pieces as needed to create a continuous level edge around the top, use extra pieces to fill in as needed.
Fill the cake with 1 cup of mousse, sprinkle with 1/3 cup of cookie crumbs, then 1/3 cup chocolate sauce. Repeat with 2 cups of mousse, 1/3 cup cookie crumbs, and a ½ cup of chocolate sauce. Place a final circle of cake on top and press down firmly. Fold over excess plastic wrap, secure tightly, and place in freezer for 2 hours, or until ready to serve and complete final step with meringue.
When ready to serve, preheat oven to 500°F, move oven rack to lower setting. Remove cake from the freezer. Whip egg whites until soft peaks form; gradually add in sugar one tablespoon at a time, sprinkle in cream of tartar and then vanilla, whip for one more minute, until stiff peaks form. Unwrap cake and invert onto an oven-safe plate and remove plastic wrap. Spread the meringue all over the cake, place in the oven for 2 minutes or until the meringue starts to turn golden brown. Remove from oven; allow to rest a couple of minutes before slicing.
**TIP** To prevent the meringue from sticking to your knife when slicing, carefully rub a drop of oil over the blade.