Preparation: grate the rinds of 1 lemon and 1 orange and pour rum and a little sugar over them. Leave them aside until it's time to use them.
Beat the egg whites to snow with a pinch of salt. Pour in the sugar to them little by little, while still beating, until you obtain a thick, fluffy mixture.
Squeeze out the juice of 1 orange and add it to the egg white mixture, along with the oil. Beat again. They will deflate a little but don't worry about that.
Sift the flour, baking powder and vanilla, add them in slowly while stirring carefully with a wooden spoon.
Next add in the the grated lemon and orange rinds (along with the rum) and stir carefully.
Pour the mixture into a suitable oiled and floured form. Bake at 356°F (180 °C) in a preheated oven for 30 min.
Squeeze out the juice of the remaining 2 oranges (or more), while the cake is baking. Those who want it sweeter can add additional sugar to the orange juice.
Pour the juice over the still-hot care and leave it to cool before taking it out of the form.
Melt the chocolate and butter in a water bath with 2 tbsp milk. Or without the water bath but lower the heat to prevent burning.
Pour the chocolate mixture over the cooled cake.
The result is remarkable, the combination of flavors superb. It's a perfect way to use leftover egg whites. I hope you like the idea!