Peel the potatoes and remove any blemishes or "eyes." Cut them into evenly sized one-to-two-inch cubes to ensure they all cook at the same rate. Place the potato chunks in a large pot and cover them with cold, salted water by about an inch. Starting with cold water ensures the potatoes cook evenly from the outside in, preventing the exterior from becoming mushy before the inside is cooked.
If you're using garlic, add the whole, peeled cloves to the pot with the potatoes. Bring the water to a gentle boil over medium-high heat. Cook uncovered for about 15–20 minutes. The potatoes are ready when they are "fork-tender," meaning a fork can be inserted into the center of a potato piece with absolutely no resistance, and the potato easily falls apart. Do not overcook, as this can lead to watery potatoes.
Once cooked, drain the potatoes in a colander. Return the empty pot to the warm burner (with the heat turned off) and add the potatoes back in. Let them sit for a minute or two, allowing the excess moisture to evaporate as steam. This is a critical step for preventing a watery final product.
While the potatoes are steaming dry, gently warm the milk or cream in a small saucepan on the stove or in the microwave. The milk should be hot but not boiling. Adding warm liquid to the hot potatoes helps it absorb better and keeps the dish at an even temperature, resulting in a creamier mash.
For the best texture, use a potato ricer or a hand masher. Avoid using a food processor or blender, which can overwork the starches and turn the potatoes into a sticky, gluey paste. Add the melted butter to the dry potatoes first and begin mashing. Once the butter is incorporated, gradually pour in the warm milk a little at a time, continuing to mash until you reach your desired creamy consistency. Be careful not to overmix.
Gently stir in salt and freshly ground black pepper. Taste the mashed potatoes and adjust the seasoning as needed. For an extra touch of richness, top with another pat of butter before serving. Serve immediately while hot.