In a 6-quart slow cooker combine potatoes and broth. Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours, adding cauliflower the last 30 minutes of cooking time.
Add buttermilk, butter, salt, and pepper. Mash until potatoes are smooth. Stir in cheese. If desired, top with bacon. Serve immediately or keep warm, covered, on warm or low up to 2 hours.