Preheat the oven to 180°C (350°F).
In a saucepan, melt down under low heat the chocolate with the butter and the milk.
Once the chocolate is melted, take it out from the heat and add the yolks, one at a time, incorporating them well each time.
In a salad bowl, mix the flour, yeast, sugar and the salt.
Add that dry ingredient mixture to the melted chocolate, and also the rum.
Whip the egg whites, then, softly add them to the mixture.
Pour in the mold and bake it for 40 min.