Preheat the oven to 180°C (approximately 350°F). Line the bottom of a 24 cm (approximately 10-inch) springform pan with parchment paper. Separate the eggs. In a bowl, beat the egg yolks and sugar until frothy. In a separate bowl, beat the egg whites with salt until stiff. Alternately fold the egg yolks and the flour into the egg whites. Scrape the batter into the pan and bake until a toothpick inserted in the center comes out clean, about 40 minutes. Release the sides of the pan and immediately invert the cake onto a wire rack. Remove the parchment and let cool.
Toast the almonds and walnuts in a dry skillet until fragrant. Let cool and chop coarsely. Grate chocolate finely and chop candied fruit.
In a bowl, whip the cream with powdered sugar until stiff and fold in the ricotta. Set aside 1/3 of the mixture. Fold the cocoa powder, nuts and chocolate into the remaining mixture. Stir the candied fruit into the reserved mixture.
Trim dark crust from the cake and then cut into two layers (one-third and two-thirds). Brush the thicker layer with liqueur and press into a dome shape in an 8-inch bowl. Spread ricotta cream over cake, then fill wiht the dark chocolate mixture. Press remaining cake layer on top (cut to fit as needed). Cover with plastic wrap and refrigerate overnight.
To serve, unwrap cake, invert onto a cake plate and dust liberally with cocoa powder and then a little powdered sugar.