Rinse the potatoes thoroughly under warm water and place on a layer of paper towels, pat dry.
Peel the potatoes until no skin remains and place them into a large colander and rinse under cold water.
Place the potatoes on a cutting board and slice in half. Transfer to a large stock pot or dutch oven and cover with COLD water.
Place the pot on the stove and bring the potatoes to a boil and cook for 15-20 minutes until fork tender.
Meanwhile, heat the milk in the microwave or on a small saucepan on the stove just until warm.
Remove the pan from the heat and drain the water from the potatoes. Rinse the potatoes under cold gently running water, drain, then return them to the pot.
Use a potato masher (or hand or stand mixer) to start breaking up the potatoes.
If using a hand (or stand) mixer, turn it on low speed and mix while slowly pouring in the warm milk.
Add in both sticks of butter, the sour cream, and seasonings and continue mashing or mixing on medium speed for 1-2 minutes until smooth and creamy.
Serve immediately. Store leftovers in an airtight container in the refrigerator for up to 3 days.