1. Whisk egg whites. When the peaks start forming add the vinegar, to stabilize the whites, and continue whisking until stiff.
2. In another bowl, beat the egg yolks with the sugar, water and vanilla until light. Add melted butter and continue beating for another minute. Then sift in the flour and mix.
3. Add the milk (it has to be lukewarm, otherwise the butter will harden) and beat until well incorporated.
4. Add about 1/4 of the egg white mixture and whisk to lighten the batter. Then gently fold in the rest of the egg whites using a spatula.
5. Grease and dust with flour a 8x8 inch square pan or a 9 inch round pan. Do not use spring form because batter is too liquid and the pan could leak.
6. Pour the batter into the pan and bake in preheated 325F oven for about 60-80 minutes or until the top is golden.
7. Cool in the pan for at least 3 hours. Serve sprinkled with powdered sugar.