Does anyone know how to make powered buttermilk, what is used and in what amounts?

In America they have powdered buttermilk in the grocery store. I think people use it for baking mostly. My grandma makes buttermilk for baking by adding a little vinegar to regular milk. I would suggest you google powdered buttermilk, I bet you can order it online

You can make a pretty good substitute for powdered buttermilk by mixing 1 cup of powdered milk with 4 1/2 tsp cream of tartar.

Baking recipes that call for powdered buttermilk are usually counting on the acidity of the buttermilk to trigger a reaction with baking soda, and so release CO2 to leaven the dough or batter. The cream of tartar provides that acidity in the substitute.

How much of this substitute you will use in a recipe will depend on how much baking soda it’s reacting with. Use too much and the goods will taste too tart; use too little and the goods will taste soapy. Best to stick to the recipe, for the first attempt at least.