I've read that some people will substitute milk for the water called for in a box cake mix (i.e. instead of using 1 1/3 cup water, use 1 1/3 cup milk). I've also read about using buttermilk for the box cake mix. I have the powdered buttermilk and I was wondering if I could pour the regular milk into my mixing bowl with my other ingredients and add the powdered buttermilk or is the powdered buttermilk only good when using water? Thanks.

Only use powdered buttermilk if the recipe calls for buttermilk. You can use either water or milk to make the buttermilk from powder, milk adding a bit of fat, but that should only make the recipe a tiny bit richer, ie tastier.

Do not use buttermilk if the recipe calls for milk or water, unless you adjust your acid I believe, because buttermilk, even the powdered stuff, contains acid/vinegar which in combination with baking soda will cause unfavorable results.

I hope that answered your question.