New Recipes, 1955

"...Using Instant PET as a dry ingredient, you can easily give your family extra bone-and-muscle-building nourishment-- much more than if regular bottled milk were used..."

Front and back covers (above), plus two pages (below) from a 32-page booklet published by Pet Milk Co., St. Louis, Missouri.

I assume that the real reason people cooked with the powdered stuff was because it was cheaper than fresh. This idea is reinforced by the bottom left page, which implies that its target audience only has the cash for one kind of baking implement. (Maybe the sequel showed how to morph a cookie sheet into a cake pan by embedding it in tinfoil or something.)