I got my organic barley grains Whole Foods's bulk aisle; though, you can certainly procure whole grains at any feed store or brewing supply shop.
How to Make Malted Milk Powder
This can be made from whole wheat or barley berries. For this experiment, I am using barley berries. You'll want to find unhulled berries because you want these to sprout. Do not use cracked, pearled, or hulled berries.
Sprout. Take 1 cup of dry grain and rinse it thoroughly in clean pure water. Soak it over night, or about 8 hours, in a 1 quart mason jar with a loose lid. Place a clean mesh lid on the top of the jar and strain out the soak water. Fill the jar again with pure water, stir and strain again. Drain. Rinse your jar of grains once a day. Watch for sprouting hairs after day one, it usually takes between 1-2 days depending on the room temperature.
Ferment. As the barley germinates, it starts to break down the starch into sugars. This is what gives malt powder its natural sweetness. If wild yeast is introduced as the barley is fermenting, the yeast will eat the sugar and convert the liquid to alcohol. You don't want the latter to occur.
Dry. Three possible methods of drying. Air/sun drying (requires 90+ degree temperatures for a few days), food dehydrator, or oven drying. This is to stop the fermenting process.
Air drying - spread moist, freshly sprouted grains on sheet pans in an even layer. Place pans in a dry, well-ventilated area. Dry for 18-48 hours depending on the humidity/temperature. If you are drying this outside, cover your grains with cheesecloth or food-safety screens to keep bugs/birds from eating it.
Food dehydrator - Set dehydrator to 115 F and dehydrate grains for 12-24 hours, or until the grains are dry.
Oven drying - Set oven to its lowest setting, between 150-170 F. Check for dryness in 8-12 hours.
When fully dry, the grains should weigh about the same as what it weighed before sprouting.
Grind. A this point, you are essentially making barley flour from sprouted, dried barley grains. You can grind these in a grain mill, food processor, or blender. You want it to a fine powder, with the granular consistency of say.. cocoa powder.
Storage. Store sprouted flour in an airtight container in the freezer or refrigerator. If using in baking, this can substitute regular flour at a 1:1 ratio.
Now that you have barley malt powder, you can blend it with dry powdered milk!
Here are some sweetened and unsweetened powder ratios.
Depression Era Malted Milk (ratio comes from backtobasicsgal blog):
6 tbsp barley malt powder
1/4 c granulated sugar
2 c powdered milk
Unsweetened Plain:
2 c instant dry milk, whole or nonfat
6 tbsp malt powder
Unsweetened Chocolate:
2 c instant dry milk powder, whole or nonfat
6 tbsp malt powder
6 tbsp unsweetened cocoa powder
To make one cup: add 1/3 c dry mix to 8 oz cold milk. Blend or whisk together. Sweeten to taste with sugar, honey, or stevia.