Prick the skin of the potatoes all over. Place potatoes on a microwave-safe plate and microwave on high power, flipping halfway, until easily pierced, about 12 to 14 minutes. Let sit until cool enough to handle. Peel and pass through a ricer into a medium bowl; set aside.

In a 6-quart pot, cook bacon over medium heat until crisp and fat is rendered, about 12 minutes. Transfer bacon to a paper-towel lined plate to drain, leaving 3 tablespoons bacon fat in the pot.

Add onion and garlic to the pot and sauté over medium heat until very soft but not browned, about 8 to 10 minutes. Add milk, season with salt and pepper. Bring to a boil, reduce to a simmer, and cook until flavors meld, about 10 minutes.

Using an immersion blender, purée onion, garlic, and milk until smooth. Add riced potatoes and sour cream, stirring to combine. Warm through and season to taste with salt and pepper. Serve garnished with sour cream, chives, cheddar, and potato chips if desired.