Separate the whites from the yolks, beat the whites to fluffy snow with a mixer.
Melt the butter and leave it to cool. To the set aside yolks add the sugar and water, beat to a homogeneous liquid mixture.
While beating nonstop with the mixer, pour the warm milk in a thin trickle to them, then the flour and vanilla, beat until it becomes a smooth mixture.
To this mixture add the egg whites slowly and carefully with a spatula, stirring until homogeneous. Pour the mixture into a small tray, covered with baking paper and greased with a little soft butter.
Put the tray in a preheated 356°F (180 °C) and bake for 1 hour. After it browns, put aluminum foil on top to prevent burning.
Sprinkle the cake with powdered sugar and leave it to cool completely. You're in for a big surprise after you cut it into slices.