The traditional answer is: both, but they serve different purposes.
Butter
- Adds richness, flavor, and a silky texture.
- Coats the potato starch, helping create a smoother mash.
- Most chefs consider butter the more important ingredient.
- If you can only choose one, butter usually gives the better result.
Milk
- Adds moisture and helps adjust consistency.
- Makes potatoes lighter and easier to whip.
- Warm milk is preferred because cold milk can cool the potatoes and make them gluey.
Best Practice
- Mash hot potatoes.
- Mix in butter first so it melts and coats the potatoes.
- Add warm milk (or cream) gradually until you reach the desired texture.
Different Styles
- Rich, restaurant-style: lots of butter, a little cream.
- Light and fluffy: moderate butter, more milk.
- Ultra-luxurious: butter + heavy cream + sometimes cream cheese or sour cream.
- Rustic: butter only, leaving some chunks.
A common ratio for about 2 lb (900 g) of potatoes is:
- 4–8 tbsp butter
- ½–¾ cup warm milk or cream
The reasoning behind “most cooks would pick butter” comes down to what each ingredient contributes:
- Butter changes the flavor dramatically. Potatoes are relatively mild, and butter adds richness, aroma, and a creamy mouthfeel that people strongly associate with good mashed potatoes.
- Milk mainly changes consistency. It loosens the mash and makes it easier to stir, but it contributes much less flavor than butter.
If you compare the two extremes:
- Mashed potatoes with butter but no milk: Rich, flavorful, slightly denser.
- Mashed potatoes with milk but no butter: Softer and moister, but often less satisfying and less flavorful.
That’s why many chefs follow the order:
- Add butter first.
- Add milk, cream, or another liquid only as needed for texture.
There are exceptions. Some people prefer:
- Butter only for intensely potato-forward flavor.
- Milk only if they’re reducing fat.
- Cream, sour cream, crème fraîche, or cream cheese instead of milk for extra richness.