For the meringues: Preheat the oven to 200 to 250 degrees fahrenheit. Line a baking sheet with parchment.
In a large bowl, beat the egg whites with an electric mixer until frothy. Add the cream of tartar and continue beating until soft peaks form. Lower the speed and gradually beat in the sugar. Add the amaretto and vinegar and beat at medium speed until stiff, glossy peaks form, about 8 minutes longer. Using a large kitchen spoon and your barely damp fingers, shape the meringue into twelve 4x3-inch ovals on the parchment-lined baking sheet.
Place on the lowest rack in the oven and bake for about 45 minutes, until golden brown. Cool on wire racks. Store in a tightly closed plastic container at room temperature.
For the Genovese Butter Cake: Preheat the oven to 350 degrees fahrenheit. Butter and flour an 8-inch springform pan.
In a large metal bowl, beat the eggs, yolks and sugar with an electric mixer on high speed until tripled in volume, about 7 minutes.
Place the bowl over a saucepan of water, bring the water to a simmer, and whisk for 1 to 2 minutes to warm the eggs slightly. Remove from the heat.
Sift the flour and baking powder together and fold into the egg mixture. Transfer one-eighth of the batter to a small bowl and whisk the melted butter into it. Pour the cake batter into the prepared cake pan.
Bake for 1 to 1 1/4 hours, or until a wooden skewer inserted into the center comes out clean. Remove from the oven and cool completely on a wire rack. (The cake can be made a day ahead if tightly wrapped in plastic.)
For the Zabaglione Filling: Make a water bath by filling a large deep pot half-full with water. Combine the yolks, cream sherry, sugar and orange zest in a large bowl (preferably copper, to get more volume). Place the bowl over the water and, while whisking the eggs vigorously, bring the water to a simmer over medium-low heat. Continue whisking for about 5 minutes, or until the mixture thickens and is light and fluffy. Remove the bowl from the heat and let cool. (Leave the water bath over low heat.)
Place the gelatin sheet, if using, in cold water to soften. When soft, squeeze out the excess water, then whisk into the egg mixture. Or, for powdered gelatin, put 2 tablespoons cold water in a small dish and sprinkle the gelatin over the water. Let stand for 1 minute to soften, then whisk into the egg mixture. Return the bowl of egg mixture to the simmering water bath and whisk constantly over medium-low heat until the gelatin is fully dissolved, about 2 minutes. Be careful not to overcook the eggs, as they may curdle, and make sure when whisking to bring all of the mixture up from the bottom of the bowl. Refrigerate until cold, whisking occasionally, then fold in the whipped cream.
For the Cream Frosting: In a medium bowl whip the cream with the sugar, vanilla, and brandy to stiff peaks. Set aside in the refrigerator.
For the Simple Syrup: Over medium heat, dissolve the sugar in 1/4 cup water and the sherry.
To Assemble: Place the cake on a cake stand. With a long sharp knife, trim off the very top (about 1/8 inch) of the cake. To slice the cake into 3 equal rounds, score 2 evenly spaced lines all around the sides of the cake. Cut into the score lines about 1 inch deep all the way around the cake, then make a clean cut all the way through the cake to separate it into three layers. With a very sharp knife, trim off the outer brown edges of each layer.
Place the top layer of cake on a flat serving plate. Place the cake ring around the cake. Dip a pastry brush into the simple syrup and brush it generously onto the cake layer to moisten it. Spread about half of the zabaglione on top. Add the next layer of cake, brush with the syrup, and top with the remaining syrup and refrigerate for at least 30 minutes, or overnight, to set.
Remove the ring from the cake. Frost the whole cake with the cream frosting, spreading it very evenly and smoothly. (Use a cake comb if you have one.) Sprinkle the amaretti crumbs on top. Dust the tops of the meringues with cocoa, then press them lengthwise into the sides of the cake, spacing them evenly. Serve immediately, or refrigerate until ready to serve, or overnight.