Preheat the oven to 180 degrees Centigrade.
Grease and flour an 9×13 inch sheet pan OR ceramic dish.
Separate the egg whites from the egg yolks.
Using a stand mixer or hand held beater, whisk the egg yolks, vanilla extract and sugar until smooth and double in volume.
Transfer the mixture to a separate bowl.
Sift the flour, baking powder and salt together.
Fold this sifted mixture into the whipped egg yolk mixture. Be gentle and make sure you do not over mix.
Clean and dry the stand mixer bowl OR a glass bowl.
Pour a drop of vinegar into this bowl and spread it with a kitchen towel.
Using the whisk attachment, whisk the egg whites on high, until stiff peaks form and the mixture looks foamy. This will take about 4-5 minutes.
Gently fold in the egg whites into the egg yolk and flour mixture.
Pour the batter into prepared pan and bake for 18-20 minutes or till the top is golden.
Let cool completely before proceeding with the milk mixture.