Question
Live in Des Moines, Iowa. Was looking at otherwise healthy leaves on younger maple tree and noticed small, "wart like" growths on the botton side of some of the leaves. About the size of a "beebee" or smaller, black and elevated on a little stem. When they are crushed, they powder. Tree looks healthy and has shown good growth but was a little concerned when I saw these growths.

Answer
These are leaf spot caused by a fungus. I am going to guess it is Phyllostict leaf spot. A leaf spot that infects late in the growing season causes very little damage except maybe the leaves will fall earlier than normal. If you want to control the fungus the leaf surface would need to be sprayed with a fungicide in the spring. A large tree would be difficult to spray and in my opinion not worth the effort since the damage to the health of the tree is slight.
Phyllosticta Leaf Spot of Maple. Pathogen: Phyllosticta minima

Like many fungal leaf spots, this disease affects a number of maple species, most prominently Amur, Japanese, red and silver maple. The disease causes little damage because the infection is localized. Spots are roughly circular and develop into tannish spots with purple to red borders. Later in the season the spots often contain black fruiting bodies of the fungus arranged in rings inside the lesion. Although this disease is quite noticeable in the landscape, especially on silver and red maples, and causes concern among homeowners, damage is minimal and fungicides are rarely necessary. If fungicides are required use a labeled product containing mancozeb or chlorothalonil.  The fungicide is only effective in the early spring and will need to be sprayed on all foliage several times at a two week intervals. Since the spots really do not harm the tree there is no need to treat for the spots.

It could be a leaf gall insect--a tiny wasp that lays its eggs on the leaf and the cells of the leaf form a gall over the eggs. Galls on leaves usually are soft and contain a liquid and the small insect egg or larvae. Since you said these were dry it is more than likely a fungi spot.

Really no need to treat either of these -fungi leaf spot or gall leaf spot. The tree may loose a little growth and may drop its leaves a little sooner than normal but the overall health of the tree is not effected. I would not worry about these leaf spots.