Put the potatoes in a large pot and cover with salted water. Bring to a boil, reduce the heat to a simmer, cover and cook until fork-tender, 15 to 20 minutes. Drain and let cool completely. Slice the potatoes into bite-size pieces.
In a large bowl, combine the mayonnaise, buttermilk, Parmesan, chives, parsley, garlic powder, onion powder and pepper and mix until well blended. Taste and season with salt if needed.
Add the potatoes to the bowl along with most of the bacon and scallions (reserve some of the bacon and scallions for a garnish) and gently mix. Transfer to a serving bowl or cover with plastic and refrigerate until ready to serve. Garnish with the reserved bacon and scallions before serving.