This cake can be made a day in advance. The Hot Chocolate Sauce can be stored, covered, in refrigerator up to 1 week; reheat over low heat until warmed through. Make both the cake and sauce with the best quality bittersweet chocolate you can find. Substitute semisweet chocolate if you wish.
Preheat oven to 350°. Grease a 9-inch springform pan; line bottom with parchment paper. Grease and flour parchment, tapping out excess flour; set aside.
Stir together chocolate and butter in a heavy saucepan over low heat until smooth. Remove from heat.
Beat egg yolks and 3/4 cup sugar in a large bowl at medium speed with an electric mixer until pale and thick. Add melted chocolate mixture, beating until blended. Add flour, beating until blended, stopping to scrape down sides. Stir in zest, chopped clementines, and orange liqueur.
Beat egg whites in a separate bowl at medium-high speed with an electric mixer until foamy. Add salt and remaining 1/4 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Fold one-third egg white mixture into chocolate mixture; repeat twice. Pour mixture into prepared pan.
Bake at 350° for 50 to 55 minutes or until slightly firm to touch. (Cracks will form on top of cake.) Remove cake to a wire rack; run sharp knife around edge of pan to loosen. Cool slightly.
Remove sides of springform pan. Invert cake onto serving plate, and remove springform bottom and parchment paper. Serve warm with Hot Chocolate Sauce; garnish, if desired.
For testing purposes, we used Grand Marnier for orange liqueur and Perugina Bittersweet Chocolate for the bittersweet chocolate. If you don't want to use orange liqueur, you can substitute orange juice. After zesting clementines, use a serrated knife to remove remaining peel and white membrane from fruit, and then coarsely chop peeled clementines. For easy serving of this cake, slice using a hot, thin-bladed knife, cleaning the blade between cuts.