Prep potatoes: Chop into evenly sized pieces. Peeling is optional.
Boil: Cover potatoes with water by 1 inch and salt generously.Bring to a boil, then cook until tender — about 6 minutes after boiling (≈15 minutes total).
Prep enrichments (optional)While potatoes boil, whisk sour cream and egg in a small bowl. Set aside.
Drain: Reserve 1/2 cup cooking water, then drain potatoes.
Start the mash: Return potatoes to the pot with the burner off. Add butter and milk; let butter start to melt.
Mash: Mash to your preferred texture. Stop early for fluffy potatoes; mix longer if you like them smoother.
Mix Enrichments (optional but recommended)Whisk 1–2 Tbsp reserved potato water into the sour cream mixture to gently warm it. Stir in nutmeg, white pepper, and salt, then add to potatoes. Shortcut: Add everything directly to the pot and stir quickly.
Adjust & Serve: Taste and adjust seasoning. Add reserved potato water as needed for texture. Serve warm.