Is phoscao a distant cousin of Bosco, the old-school chocolate syrup produced to this day in Towaco, New Jersey? I haven't the slightest bit of direct evidence, but I note that the trade name Boscul — which the Jersey manufacturer also used for coffee, tea, and peanut butter — for some reason took a tweak when the company introduced its syrup.
Bosco was invented in 1928 by a physician whose name has been lost to history. At the time, Phoscao had been a brand name for a French chocolate powder for many years; during World War I, an advertisement proposed that for coffee drinkers mired in the trenches, "happy are those who can mix in a spoonful of Phoscao, thus making a breakfast stimulating and substantial. So, in your next package mailed to the front, add your Phoscao."
The eponymous Phoscao company was acquired by a Spanish manufacturer in 1964, and the brand name itself has apparently been retired. Immaculee, a Haitian bakery, seems to be using the name in a generic sense for this chocolate milk (about 12 fl. oz.; $3), appealing to childhood memories born in the French-speaking Caribbean.
Phoscao, Bosco — I can say no more, except shake well before serving.
Immaculee II Bakery
1411 Nostrand Ave. (Linden Blvd.-Martense St.), Prospect Lefferts Gardens, Brooklyn