Peel potatoes and cut into even 2-inch chunks. Even sizes = even cooking.
Start in cold salted water — this prevents the outside from overcooking before the inside is done. Bring to a boil, then reduce to a steady simmer.
Cook 18–22 minutes until a knife slides through with zero resistance. Any firmness means they need more time.
Drain completely. Return to the pot over low heat and shake for 1 minute — the heat drives off surface moisture, preventing watery mash.
Rice the potatoes (or use a masher — never a blender or food processor, which makes gluey potatoes).
Warm butter and heavy cream together in a small saucepan until butter melts. Warm dairy = fluffier mash. Cold dairy makes dense mash.
Fold the warm butter mixture into the potatoes in three additions, stirring between each. Season generously with salt. Taste. Add more butter if in doubt.