Preheat oven to 325 F degrees. Grease and line an 8 x 8 inch baking dish with parchment.
Separate eggs and add the egg whites to a mixer and beat the egg whites stiff. Place egg whites in a bowl and set aside.
Beat the yolks & sugar until light. Add butter and vanilla. Beat for two minutes. Add flour and mix it in until fully incorporated.
Slowly start adding the milk and beat until everything is well mixed together. Add the egg whites, one third at a time and gently fold them in using a spatula, repeat until all egg whites are folded in.
Pour batter into baking dish and bake for approx 60 minutes or until the top is lightly golden. Cool and dust heavily with powdered sugar.