Peel potatoes and cut into 1-inch pieces.
If boiling potatoes in Instant Pot, place potatoes into inner pot and cover with water. Set pressure valve to Sealing. Cook on High Pressure for 30 minutes then let pressure release naturally. Potatoes should be fork-tender.
If boiling potatoes on stovetop, place potatoes in large stock pot and cover with water. Cover and bring to a boil. Boil for 45 minutes or until potatoes are fork-tender.
Drain potatoes and move to a mixer-safe bowl or pot (if potatoes cooked in a stainless steel pot, you can use the same pot for mixing.) Add butter then cover for a few minutes until butter softens and starts to melt.
Using hand-mixer, mix on slow speed 2 minutes until potatoes are broken up. Add salt and pepper, to taste. Slowly add evaporated milk. Turn mixer to high speed and continue mixing until potatoes are smooth and creamy.