Separate the egg yolks from the whites.
Melt the butter and the chocolate. Set aside. Mix the egg yolks and ¼ cup of sugar until smooth.
Beat the egg whites with the rest of the sugar (¼) cup until firm.
Add the butter-chocolate mix to egg yolk-sugar mix. Stir until everything is blend in.
Add the flour gradually and stir.
Add the beaten egg whites and mix it with a rubber spatula, be careful to mix by starting from the bottom and bring it up and repeat. Not to "break" the whites.
Cook in the oven at 350F for 25 minutes - your knife should come out clean when you dip in the cake to know if it is cooked.