Add the tablespoon of water & the drizzle butter in, continuing to beat for a minute or two.
Add the flour, fully incorporating it.
Slowly add the milk & continue beating on low until everything is well mixed.
Pour the egg/butter/milk mixture into a bowl & set aside while you go on.
IMPORTANT: Wash beaters & bowl (if using same) VERY well, to ensure not a speck of butter or egg yolk remain.
Whip egg whites until stiff.
GENTLY, whisk the egg whites into the egg yolk mixture, a third at a time, until all egg whites are fully folded in .
Pour into greased 8 x 8 baking pan.
Bake 60 - 70 minutes, until top is lightly golden.
Cool completely, sprinkle with powdered sugar.
Cut to desired size.
Note: Yes, you could whip the egg whites prior to beating the yolks, setting them aside, and adding to the yolk mixture, when ready. Just be aware there is the possibility that the whites may separate while they are waiting.