Pre-heat the oven at 140 degrees Celsius (284 Fahrenheit).
Melt the butter, then leave to cool.
Separate the egg yolks from the egg whites.
Beat the egg yolks with the icing sugar until you get a smooth, light-coloured texture.
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 with non-stick perchament paper, then add the batter.
Bake in the oven for one hour, 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 some more sugar on top of the cake and serve it cold.