Preheat your oven to 170 degrees and line a 20cm square cake tin with baking paper, making sure to leave plenty of paper hanging over the sides.
Separate your eggs and place the egg whites into a CLEAN and DRY bowl - this is very important and beat using an electric mixer until stiff peaks form. Set the egg whites aside until needed.
Place the egg yolks and caster sugar together in a large bowl and use an electric mixer to beat until they have combined and are light and frothy.
With the electric mixer still going on a low speed, gradually add the melted butter and vanilla extract before adding the sifted flour a few spoonfuls at a time.
Gradually add the lukewarm milk and continue to beat on a low- medium speed to combine all of the ingredients.
Add the egg whites to the bowl and gently fold together the mixture.
Pour the batter into your prepared cake tin and place into your preheated oven and cook for 35 minutes or until a golden crust has formed on top of your cake.