This dressing can be made quickly in a jar.  I have frozen butter milk, but it become very thin. There is also powdered buttermilk in the store.  Fresh works best.

Ingredients: 

• 1/2 cup buttermilk, well-shaken
• 1/3 cup mayonnaise (this is more than is in the original, to thicken the dressing further)
• 2 tablespoons cider vinegar
• 1 tablespoon sugar
• 3 tablespoons finely chopped red onion

Directions:

Mix the dressing ingredients in a jar with a lid, with a good pinch of salt and black pepper.

Serves:  Makes about one cup, enough for six cups of slaw

Source:  Smitten Kitchen