Preheat your oven to 180 C / 350 F
Butter and flour a 8 inch square cake pan . You can line the bottom with parchment (optional)
In a bowl , whisk together flour , baking powder and salt. Keep aside.
In a saucepan , heat together butter and milk until butter melts . Keep this warm until needed
In a large bowl , beat together the eggs , sugar and vanilla
Beat well until mixture is tripled in volume and is light and fluffy.
Beat or fold in the flour mixture at low speed until all the flour is mixed in. Scrape the sides and bottom of bowl to ensure even mixing.
Beat in the hot milk-butter mixture until smooth. This will take only a few seconds
Pour batter immediately into prepared pan and bake for 25 to 30 minutes until top is set , springs back when pressed and maybe even lightly browned
Remove from oven and cool in pan for 5 minutes.
Gently remove from pan , remove parchment (if using) and cool right side up on a wire rack until cool