I considered the matter of volume too, and my theory was something like this; since the recipe calls for 1 cup regular milk, just substitute buttermilk for both the regular milk and vinegar. Some math will be required to figure out the amount needed, but that shouldn't be a problem.
One batch...
I have a muffin recipe which uses works on the vinegar+baking soda principle, and I'm wondering if I could substitute the vinegar with buttermilk instead. One resource suggests using a ratio of 1 teaspoon vinegar to 1 cup buttermilk. How would this affect the the finished product?
Thanks.