If you use powdered buttermilk, whisk in the powder first, and then slowly add a cup of water.
Then add the seasonings. Whisk again.
Or you can make it in the blender by putting everything into the blender and whirling until smooth.
The mixture will be a little thin, but it will thicken on standing. Store in a pint size jar, makes 2-cups.
Keep in the refrigerator for a few weeks. Tastes really good with yogurt by the way. This is my favorite way of making it. It tastes good with powdered buttermilk too, but the consistency is thinner than with yogurt.
Note: If desired, you may add a tablespoon of Parmesan Cheese and/or a tablespoon of lemon juice to the dressing in addition to the seasonings above.