Pre-heat the oven at 140 degrees Celsius (284 Fahrenheit).
Melt the butter, then leave to cool.
Melt the chocolate, and leave to cool.
Separate the egg yolks from the egg whites.
Beat the egg yolks with the powdered sugar until you get a smooth, light-coloured consistency.
Add the cocoa powder and melted chocolate, and mix again.
Gradually add the flour, one tablespoon at the time, and beat until fully incorporated, then add the vanilla extract, butter and milk, and beat again.
Beat the egg whites separately until you get stiff peaks, then use a spatuala to gently fold them into the batter.
Line a large rectacle baking tray ( 15x26 cm, 6x10 inches )with non-stick perchament paper, then add the batter.
Bake in the oven for 70 minutes, or until the top is golden brown, and a tooth pick inserted in the middle comes out clean.
Turn off the oven, then leave to cool completely in there. Once it's cool, refrigerate overnight before slicing.
Optionally, you can powder the cake with icing sugar or cocoa powder before slicing it.