Preheat the oven to 150°C/300°F/Gas 2. Grease a 20x20cm square baking dish.
Beat the egg yolks with the sugar until pale light. Add the melted butter and vanilla extract and continue beating for another minute. Add the flour and the salt and mix until fully incorporated. Slowly start adding the milk and beat until everything is well mixed together.
Whisk the egg whites until stiff. Add the egg whites to the main mixture, a little at a time, and gently fold them. Do not over break the whites completely as you still want some lumps in the batter.
Pour the batter into baking dish, level the top with a spatula and bake for 50 minutes. The mixture won’t be completely set, the top will still be wobbly. Let cool and refrigerate for at least 2 hours, or overnight.
Dust with icing sugar before serving.