Set the oven to 450°F and allow it to heat fully. A hot oven helps the buttered potato surfaces brown rather than linger pale and damp.
Put the potatoes in a saucepan with a generous pinch of salt and enough water to cover them by 1 inch, then bring to a boil over medium-high heat and cook for 15 to 20 minutes before draining. A fork should enter the center easily, with no firm resistance.
While the potatoes cook, fry the bacon in a large skillet over medium heat for 5 to 7 minutes, turning occasionally, then let it cool before chopping and reserving it. The slices should be crisp enough to break into crunchy pieces once cooled.
Spread 2 tablespoons melted butter across two sheet pans, divide the potatoes between them, press each very thin with the flat bottom of a glass, then brush with the remaining 2 tablespoons butter and season with 1 teaspoon each salt and pepper. Aim for irregular flattened circles with ragged edges and space between them.
Roast for 25 to 28 minutes, then take the pans out and leave the potatoes to rest while preparing the dressing. Look for deep golden surfaces and crisp, browned edges rather than color alone on the highest points.
Stir together the mayonnaise, sour cream, dill, parsley, chives, and garlic in a large bowl, season with a pinch of salt and pepper, then gently fold in the potatoes, half the bacon, and half the roasted peppers before transferring to a serving bowl and topping with the remaining bacon and peppers plus extra dill and chives. The dressing should lightly coat the potatoes, with the colorful garnishes visible across the top.