Buttermilk - dairy free

2-to 8 teaspoons Lemon juice, apple cider vinegar, or cream of tartar

Plain or unsweetened milk alternative (almond, rice, soy)
 
Directions: Add enough of the milk alternative to the juice, vinegar, or cream of tartar to make one cup. Let the solution stand for ten minutes before adding to your recipe.

Yield: equivalent of 1 cup buttermilk

Source:

http://www.godairyfree.org/Recipes/Milk-Subs/Baseline-Buttermilk-Alternative.html