First, prepare the chocolate meringue, which will form the base of the cake. Beat the 8 egg whites until they are stiff. Add the sugar and add a teaspoon of cornstarch to prevent the foam from going down.
Now, add the unsweetened cocoa to the meringue and whisk until the mixture is smooth.
Preheat the oven to 392°F. On parchment paper, spread the meringue into three circles, which will become the three layers of the cake. Bake all three parts at 266°F until they are the desired consistency.
For the filling and mousse, add the cream to a saucepan, stirring well. When it begins to boil, add the chocolate, mixing well. Use 1/3 of this mixture for the ganache, adding the oil and a tablespoon of water to it, mixing well.
The remaining 2/3 of the chocolate will be used to make the mousse. Beat 3 egg whites and add the powdered sugar. Add this mixture to the chocolate.
To assemble the cake, place the first and second meringue layers on a rack, covering with a layer of mousse. Top with the third layer and pour the ganache on.