Place chopped potatoes in large pot and cover with cool water. Add teaspoon of kosher salt. Over high heat, bring to a boil and continue to cook until the potatoes are very tender, about 15-20 minutes. Drain.Alternatively, place potatoes in Instant Pot with one cup water and cook on high pressure for 12 minutes. Drain.
Meanwhile, cut stem end (opposite of root end) off head of garlic to expose tips of cloves. Remove outer papery skin, but leave inner skin on. Place in an oven safe dish. Drizzle with olive oil. Roast in 400 degree F oven until golden brown and fragrant, about 20 minutes.
Once garlic has roasted and is cool enough to handle, separate cloves from any of the skin. With the side of a large knife, smash cloves into a paste.
In bowl of stand mixer, combine cooked potatoes, smashed roasted garlic, butter, and sour cream. Mix on low until desired consistency is reached, about 2-4 minutes.
Add salt and pepper as desired. Serve immediately.
To make ahead, prepare as directed and refrigerate. To reheat, add potatoes to slow cooker and heat on low until warm, stirring occasionally. Once warm, change slow cooker setting from low to warm. If they appear dry, you can add chicken broth or, dare I say, more butter.