Warm a large stock pot over medium high heat. Add butter to pot. Once butter starts to melt, whisk in flour. Whisk together and slowly pour in chicken stock. Continue whisking and add sour cream and cream cheese. Whisk until mixture comes together. Slice potatoes in half and scoop out the insides of the potato, add to pot. Smash potatoes into chunks using a potato masher or wooden spoon, until desired consistency is reached. I like my soup kind of chunky. Add sharp cheddar cheese, salt and pepper. Stir until cheese is melted. Serve and enjoy!