Don’t let the milk boil.
You simply want everything warm and the butter melted.
Why warm them?
Adding cold milk and butter straight from the refrigerator can cool the potatoes down and make it harder to achieve that smooth, creamy consistency.
Step 4: Drain the Potatoes
Once the potatoes are tender, drain them thoroughly.
Return them to the warm pot and let them sit over very low heat for about a minute, stirring gently.
This helps excess moisture evaporate.
That’s important because watery potatoes aren’t going to become magically creamy by adding more dairy.
Step 5: Mash!
Now it’s time for the fun part.
Add the butter first and mash the potatoes until they’re mostly smooth.
Then gradually add the warm milk.
Don’t pour it all in at once.
Add a little, mash, taste, and continue until you reach the texture you like.
Some people want mashed potatoes so creamy they practically fall off the spoon.
Others prefer them thick, fluffy, and slightly rustic.
There is no single correct texture.
Step 6: Season Properly
Add salt and freshly ground black pepper.
Taste.
Then taste again.
Potatoes can absorb quite a bit of seasoning, so don’t be afraid to adjust the salt gradually.
For extra flavor, you can add:
Roasted garlic
Chives
Parsley
Parmesan
A splash of cream
Extra melted butter
A little black pepper
The Secret to Extra-Buttery Potatoes
If you’re firmly on Team Butter, there’s absolutely nothing wrong with increasing the butter.
For an especially rich version, use around 100–125 g of butter per kilogram of potatoes.
Add it gradually and allow it to melt into the hot potatoes.
The result?
Rich, silky, buttery mashed potatoes. 
What If You Only Have Milk?
No problem!
You can absolutely make mashed potatoes with milk.
Use whole milk if you have it because its higher fat content gives you a richer result.
If you’re using lower-fat milk, you may simply want to add a little extra butter to compensate for the reduced richness.
What If You Only Have Butter?
You can also make mashed potatoes without milk.
Add butter gradually and use a little of the reserved potato cooking water to loosen the mash.
