For the chocolate cake: Beat the butter with half the sugar until soft and fluffy. Add the cinnamon, the salt and the lemon zest. Continue to beat adding the yolks one by one.
Melt the chocolate in a double boiler and, when it cools, add it in the mixture beating all the time. Whisk the whites with the rest of the sugar into a stiff meringue and fold in stirring gently. Gradually add the flour.
Thoroughly grease a round cake pan with butter and dust with flour. Transfer the mixture into the pan and bake in a pre-heated oven for 20΄-30΄ until a knife inserted in center comes out clean. Leave the cake to cool for about 2 hours. Split horizontally into two layers.
For the chocolate filling/frosting: Beat the whole eggs. Melt the chocolate in a double boiler and add the beaten eggs, the icing sugar, the butter and the rum. Give everything a good stir until well combined and allow to cool. Place one layer of cake on a serving plate and liberally spread chocolate filling over it. Cover with the second layer and spread the remaining cream over the top and sides of the cake. If desired, sprinkle with chocolate shavings for garnish.