It won't affect the taste, after all, the milk powder is just as a small portion of the ingredients. I haven't really bought baking milk powder on purpose, I've used what I have at home, baby milk powder, whole milk powder, skim milk powder. Don't worry too much, you think ah, you use chocolate, coconut, cranberries and what not are not babies, more or less a little additives. Either way making a cookie cake is much healthier than a bunch of flavors and preservatives you can buy out there.

Of course, if you don't like baking powdered milk or think it's a hassle to buy it just keep using baby powdered milk~~