Whisk ⅔ cup sour cream, ½ cup buttermilk, 1½ teaspoon garlic powder, ½ teaspoon onion powder, 1 teaspoon salt, and ¼ teaspoon pepper in a large bowl until smooth.
Halve 4 avocados and discard the pits. Scoop the avocados into the bowl with the dressing. Mash them up with the dressing to the desired chunkiness.
Stir in 2 diced Roma tomatoes, ½ cup chopped red onion, ¼ cup fresh chopped chives, and 2 tablespoons fresh chopped parsley. Season more with salt and pepper if needed.