Malted Milk Powder is a dry mixture of malted barley flour, wheat flour and powdered milk.
The malted barley and wheat flour have water added to them, and are simmered to make them a “mash” (as is made by brewers.) The simmering converts the starches into sugar. The water is then filtered, the solids are discarded, liquid milk is added, then moisture is removed by evaporation. The resulting solids are then ground into a light beige coloured powder, and packaged for shipping.
There are many commercial brands of it including Ovaltine, Carnation Malted Milk Powder, Horlicks, etc.
Eaten on its own, the powder has a somewhat bitter taste. But you can sprinkle it on ice cream, add it to milk shakes and baked goods, sprinkle it on peanut butter and banana sandwiches, etc.
You can also buy the powder chocolate-flavoured.
Malted Milk Powder is not the same as Malted Barley Flour (aka Malt Powder.)
Malted Milk Powder was originally was marketed as a health food for invalids and infants. It really took off, though, in the 1920s when soda fountains discovered the taste it could add to the drinks they made up. Chocolate malted milks, while thought of by many people as a thing of the 1950s, actually first became a craze in America in the 1920s.
One manufacturer in Wisconsin (the Elgin company, started production in 1894, sold to Borden’s in 1903), called their malted milk “Milkine”. It was a mixture of 50% powdered cow’s milk, 22% malted barley, 22 % flour, 5% beef (something), 1% lime (the chemical) and salt. They recommended that you add water, hot or cold, to make a beverage that would give you just about everything you needed for good nutrition. Borden renamed the product from “Milkine” to Meadow Malted Milk, and removed beef from the ingredients.