Mix sour cream, mayonnaise, buttermilk, salt, garlic powder, and 2 tbsp. water in a medium bowl. Stir in half the cheese, mashing with the back of a spoon to smear into dressing. Chill 30 minutes.
Set a lettuce half on each plate. Drizzle with dressing and sprinkle with remaining cheese. Top with parsley and pepper.
*A mini romaine, Little Gem lettuce has sweet, crunchy, and densely packed leaves. Find it at farmers' markets and well-stocked grocery stores.
Make ahead: For dressing, up to 3 hours, chilled.