Begin by peeling the potatoes if desired. Cut them into evenly-sized chunks to ensure even cooking.
Place the potato chunks into the slow cooker.
Pour in the chicken or vegetable broth, ensuring the potatoes are submerged.
Add the butter, garlic powder, salt, and black pepper to the slow cooker.
Cover the slow cooker and cook on low for 4 hours, or until the potatoes are tender when pierced with a fork.
After cooking, drain any excess broth. If you prefer a creamier texture, reserve a little of the broth to add back in later.
Use a potato masher or hand mixer to mash the potatoes until smooth and creamy.
Stir in the milk and sour cream (if using) until well combined. Adjust seasoning to taste.
Serve warm, garnished with fresh chives if desired.