Mix the yolks with the sugar until the mixture gets clearer and frothy ; add the melted butter, then add the flour.
Melt the chocolate in a saucepan with the milk (very gentle flame, turn non-stop with a wooden spoon) ; when melted, add it to the batter.
Whisk the egg whites with one pinch of salt using and electric mixer until they get completely frothy ; gently incorporate them to the batter with a whisk.
Butter a tin and pour the batter in ; bake in oven for 30 minutes or so.
If you want to coat this cake, melt the chocolate with the milk in a saucepan when the cake is baked (very gentle flame, turn constantly). When melted, add the sour cream and coat the cake with this cream.
You can choose to flavor the coating with orange for instance using 1 teaspoonoon of Cointreau type liquor, it is delicious (my favorite !) or using one teaspoonoon of orange-flower water...which is very good as well.