Preheat your oven to 325°F. In medium bowl, beat 4 egg whites until stiff peaks form. In separate large bowl add egg yolks, sugar, water and vanilla. Beat until light.
Add melted butter and continue beating for another minute. Add flour and mix just until well combined.
Add lukewarm milk and fold in egg whites.
Prepare an 8″x8″ pan with butter or oil. Pour batter into pan and bake for about 60 minutes until top is golden. Cool in the pan for 2 hours. Top with powdered sugar.