chocolate meringue layer cake
Preheat the oven to 275ºF (135ºC). Line two cookie sheets with parchment paper.
Using a stand mixer fitted with the whisk attachment, or a handheld electric mixer, bet the egg whites on low speed for 2 minutes, until bubbles start to form.
Increase the speed to medium-high and whip until the egg whites start to thicken, roughly 2 to 3 minutes.
Add the sugar, 1 tablespoon at a time, until it is all incorporated. The egg whites will double in volume and become shiny.
Add the cornstarch, vinegar, and cocoa powder and mix for 2 minutes more.
Spread a flat 10-inch round disc of meringue on each prepared cookie sheet.
Bake for 1 to 1 1/4 hours. Turn off the oven but leave the meringue in the oven to dry out for 3 hours.
Meanwhile, whip the cream until stiff peaks form. Refrigerate until ready to assemble the cake.
To make the three-ingredient chocolate mousse: see the instructions in the section below.
To assemble the cake: Set one meringue disc on a serving platter and generously spread the chocolate mousse over top, followed by half of the whipped cream. Place the other meringue disc on top and finish with the remaining whipped cream. Garnish with chocolate shavings (see notes in the section below). Keep refrigerated and enjoy within 2 days.
three-ingredient chocolate mousse
Put the chocolate and milk in a large heatproof bowl and gently melt in the microwave or over a bain-marie (see notes section below) or a double boiler. Set aside to cool slightly.
Using a stand mixer fitted with the whisk attachment, or handheld electric mixer, whip the egg whites until stiff peaks form.
Fold a spoonful of the egg whites into the chocolate mixture to loosen it. Gently fold in the remaining egg whites until fully incorporated, taking care not to overmix.
Keep refrigerated and covered tightly with plastic wrap until you're ready to assemble the cake.