Peel potatoes and quarter them. Place potatoes in a large pot and cover with cold water. Bring to a boil and cook until easily pierced with a knife about 15 minutes. Drain well.: The aroma of raw potatoes is faintly earthy, and when you quarter them you create uniform pieces that cook evenly. Make sure pieces are similar in size so none finish much earlier than the rest. A common mistake is leaving large gaps in size, which leads to some pieces being undercooked while others fall apart.
Add butter to potatoes and begin mashing with your potato masher. Pour in a little of the warmed milk at a time while mashing to achieve desired consistency.: Starting with cold water ensures the potatoes heat evenly from edge to center, preventing a mushy exterior and undercooked core. The sound of tiny bubbles building as the water warms is your cue that even heating is happening. If you start in hot water, the outside can overcook before the center warms through, producing an inconsistent texture.
Season with salt and pepper to taste and garnish with chives or parsley if desired.: During boiling, the water should bubble steadily but not violently; a rolling boil can batter the tubers and bruise them. Check doneness by sliding a knife into the largest piece, feeling for a smooth slide with no resistance. Overcooking will make the potatoes waterlogged and prone to becoming gluey when mashed, so watch the clock and test early.
Drain well: After the potatoes are tender, drain them completely so excess water does not dilute the flavor or thin the mash. The quiet clatter of the pot as the water leaves is the last audible sign before you begin mashing. Leaving water behind is a common error that leads to a watery, bland result, so give them a good shake in the colander.
Add butter to potatoes and begin mashing with your potato masher: As you press the potatoes, you will hear a soft, rhythmic squelch and see the texture change from chunky to cohesive. The butter should melt into warm potatoes , releasing a nutty aroma. I prefer a hand masher because it preserves a lighter texture; using a mixer can overwork the starches and make the mash gummy.
Pour in a little of the warmed milk at a time while mashing to achieve desired consistency: Warm milk integrates smoothly, leaving the mash glossy and tender. Add it slowly, watching the texture change and stopping when the mouthfeel feels right. If you pour too much at once, the mash can turn soupy, so proceed in small increments and taste as you go.
Season with salt and pepper to taste: Salt brings out the underlying potato flavor, while pepper adds a gentle counterpoint. Sprinkle small amounts, stir, and taste, because it is easy to over-season. Remember that butter already contains salt, so adjust carefully.
Garnish with chives or parsley if desired: Fresh herbs add a bright finish and a pop of color against the creamy backdrop. Scatter finely chopped chives or parsley just before serving to keep their vibrancy. Adding herbs too early can cause them to wilt and lose their fresh aroma.