Preheat the oven to 180C/360F, and arrange a baking rack onto the middle shelf.
Place the chopped chocolate and butter in a microwave-safe bowl; microwave in 30-second intervals, stirring between each until melted. Set aside until needed.
Separate the egg yolks from the egg whites, and set the egg whites aside. Whip the egg yolks with ⅔ of the sugar and orange zest, until they're tripled in volume and reach a foamy and light mixture. Then add the rum if using.
Add in the melted chocolate and butter into the egg yolk mixture, followed by the almond flour and mix with a spatula until combined. The cake batter will be dense and quite compact, and that's totally ok.
Beat the egg whites with the remaining ⅓ (50 gr) of the sugar, until stiff peak form. Use a hand mixer to make this easier.
Fold the egg whites into the batter, and mix with a spatula from the bottom upwards, trying not to disassemble the mixture.
Spray with baking oil a 7-inch springform cake pan with loose base, pour in the cake batter and even the top using a spatula.
Arrange the cake pan onto the middle rack and bake in the oven for about 40 minutes, then insert a toothpick in the middle of the cake. If it comes out just about wet and a little crumbly, your cake is done, otherwise continue to bake for a further 5 minutes, then check again. Take the cake out from the oven and allow to cool completely before removing it from the pan.
Dust generously with confectioner sugar and optional cocoa poweder, and serve.