Add potato cubes to a large pot and add enough cold water to cover. Add kosher salt and bring to a boil over high heat. Boil potatoes for 20-25 minutes, or until they easily break apart when pierced with a knife. Remove from heat and drain.
Add butter and stir to evenly distribute the butter. Mash or beat the potatoes to desired consistency. Add the sour cream and whole milk and mix together. Stir in Spiceology Black & Bleu Rub. Taste, then season with salt & pepper to your liking. Top with more butter before serving. Serve immediately.