Whisk the sour cream, buttermilk, garlic, onion powder and sugar in a large bowl until smooth; season with 3/4 teaspoon salt and 1/4 teaspoon pepper.
Halve the avocados and discard the pits; scoop the avocados into the bowl with the dressing and mash. Stir in the tomatoes, onion, chives and parsley and season with salt and pepper. Serve with chips.
You can make this guacamole 4 to 6 hours ahead; lightly cover and refrigerate. The sour cream and buttermilk will help prevent discoloration.
Photograph by Christopher Testani
Courtesy of Food Network Magazine