Peel potatoes and cut into 1-inch cubes. Rinse in cold water until the water runs clear, then shake off excess water. Add potatoes to a greased 6-quart Crock-Pot.
Add cubed butter, salt, and bay leaves over the potatoes. Pour chicken broth over everything (no need to stir). Cover and cook on high for 3.5–4.5 hours, until potatoes are fork-tender. (My Crock-Pot takes 4 hours.)
Remove the lid carefully to release steam. Drain potatoes in a colander and discard bay leaves. Let the potatoes stand for 5–7 minutes to dry thoroughly.
Dry out the Crock-Pot. Use a potato ricer to mash the hot potatoes directly back into the slow cooker.
In a separate small pot, melt butter over medium-low heat. Stir in heavy cream and sour cream until smooth, then remove from heat. Pour the mixture over the mashed potatoes and gently stir until combined.
Add salt and pepper to taste (I start with 3/4 teaspoon of each and adjust). Serve hot straight from the Crock-Pot or keep on the warm setting until ready to serve. Best enjoyed promptly.
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Notes
Note 1: Slow-cooked mashed potatoes can turn gluey or rubbery if overmixed. Overbeating releases starch, creating a pasty texture, so keep mixing to a minimum. For best results, avoid using a stand mixer, hand mixer, or food processor. A potato masher or ricer works best and helps maintain the perfect texture.Make Ahead: For best results, serve fresh. To prep ahead, peel and cube the potatoes, store them in cold water in the fridge for up to 12 hours, then drain and proceed with the recipe.Storage: Leftovers reheat okay but are best fresh.