Put the buttermilk, regular milk, and 1 cup of water into your blender; buzz a few seconds to mix them.
Add a second cup of warm water and the milk powder; buzz to mix the powder in completely.
Add the remaining water, mix, then pour the milk into a glass jar. You' will probably need a jar larger than a quart, because your blender will make about a cup of foam. Keep the mixture at 110°F until it clabbers-about 8 hours or overnight.