In a large pot, combine the potatoes and enough salted water to cover potatoes by 1-inch. Bring to a boil. Lower the heat and simmer until the potatoes are fork tender, about 10 to 12 minutes. Drain and let cool slightly.
In a bowl, whisk together the mayonnaise, sour cream, buttermilk, and mustard. Add the garlic, salt, pepper, and cayenne and whisk to combine. Place the potatoes in a large bowl and add the dressing, tossing to coat evenly. Add the eggs, onions, parsley, and bacon and toss to combine. Adjust the seasoning to taste.
Serve warm or chilled. (Potato salad should be refrigerated if not being served immediately.)