Fry the beef bacon in a skillet over medium heat until crispy. Remove from pan, chop into crumbles, and set aside. Reserve a small amount of the rendered beef fat.
In a 6-quart slow cooker, combine the diced potatoes, chopped onion, chicken broth, ranch dressing mix, garlic powder, and half of the cooked beef bacon crumbles.
Cover and cook on LOW for 6-7 hours or HIGH for 3-4 hours, until the potatoes are tender and easily pierced with a fork.
About 30 minutes before serving, stir in the softened cream cheese cubes. Whisk gently until melted and smooth. For a thicker soup, mash about 1/3 of the potatoes directly in the pot.
Stir in the heavy cream (if using) and 1.5 cups of the shredded cheddar cheese. Stir until the cheese is melted and combined.
Ladle into bowls and top with the remaining cheddar cheese, the rest of the crispy beef bacon, and fresh chives.