Melt butter in a skillet over medium heat.
Add onions, salt, pepper, onion powder, and cayenne pepper. Cook for 5 minutes, stirring.
Reduce heat to medium-low and cook onions for 20–25 minutes, stirring occasionally, until sweet and golden. Cool completely.
In a bowl, mix cream cheese, sour cream, and mayo until smooth. (Microwave for 10-second bursts if needed.)
Stir in caramelized onions until just combined.
Refrigerate for at least 2 hours, preferably overnight, to develop flavors.
Serve at room temperature, garnished with chives if desired, alongside crinkle-cut potato chips.