Preheat the oven to 350 degrees F/180 degrees C. Grease and line a 9-to-10-inch/23 to 25 cm cake pan (springform or tube pan). For the cake: Separate the eggs into two bowls. Beat the yolks with the sugar and zest, then stir in the almonds. Beat the whites to peaks. Stir a spoonful of whites into the yolk mixture, then fold in the rest, gently. Pour the batter into the pan and bake until set, about 45 minutes. Let cool slightly, then unmold onto a serving platter. For the syrup: Heat the orange juice and sugar together in a saucepan and boil for 5 minutes. Stir in the liqueur if using. Spoon the syrup evenly over the cake, letting the syrup soak in as you go (it will absorb all of it). Serve the cake with candied orange zest on top and whipped cream on the side.