From the Homesick Texan:

Buttermilk dressing, extra spicy
Ingredients:
1/4 cup of mayonnaise
1/2 cup of sour cream
1/2 cup of buttermilk
1/4 cup of chopped cilantro
1/4 cup of freshly squeezed lime juice
1 clove of garlic, crushed
1 serrano pepper, finely diced
1 teaspoon of chopped fresh chives
1/8 teaspoon of cayenne pepper
Salt and black pepper to taste

Method:
Mix all the ingredients together and let chill for an hour.
Makes 1 cup, keeps for a week in the refrigerator.

Notes: I like my dressing on the thinner side, so if you prefer it thicker use 1/2 cup of mayonnaise. And if you don't want it to be so spicy, feel free to omit the Serrano or substitute a jalapeno pepper instead. Also, you can use parsley instead of cilantro if you're one of those “Cilantro tastes like soap" people, I won't be insulted.