A classic version of a seven-layer Viennese torte - rich with a creamy chocolate ganache frosting. The cake is adapted from The Daring Kitchen.
Put the chocolate chips and the boiling water in a blender; blend until smooth. Add egg yolks, cognac (you can skip this if you are making cake for kids), and butter and continue blending until smooth. The frosting will be creamy but may appear a little runny. Pour it into a bowl and cover it loosely; refrigerate for 2 hours until it is thick and creamy.
Preheat the oven to 400 degrees. Prepare two large flat jelly roll pans (cookie pans with a 1-inch lip) by cutting a piece of parchment paper to fit the bottom of each.
Place the egg yolks together in a medium bowl. Add 2/3 cup of the powdered sugar and the vanilla to the egg yolks, and then blend with a hand mixer until the mixture is thick and pale yellow, about 3 minutes.
In a separate bowl, with clean beaters, beat the egg whites until soft peaks form. Add the remaining 2/3 cup powdered sugar and beat until stiff peaks form. Using a soft rubber spatula, gradually and gently fold the egg whites into the egg yolk mixture (some white streaks should remain). Sift together the flour, cornstarch, and salt and fold it in to the egg mixture slowly and carefully.
Divide the mixture in half and spread each half thinly on top of the parchment paper on the jelly roll/cookie sheet. Bake in the top rack of the oven for five minutes, or until just barely golden brown around the edges. Invert the cakes on a cutting board and peel the paper off the back.
Cut each sheet into three equal rectangles. Set the first rectangle on a platter and cover it with a layer of frosting. Place a second layer on top of the first, frost, and then repeat for the remaining layers.
Use the remaining frosting to cover the sides and top. Chill until ready to serve, for at least an hour. Slice in 1/2-inch thick rectangles.