Place the diced potatoes in a pan. Cover with water. Bring to the boil, lower the heat and simmer until very tender (as in, when pierced with a fork, the potato will break apart).
Drain the potatoes and transfer to a bowl. Whether using a fork or a potato ricer, do the mashing while the potatoes are hot.
Add the butter to the mashed potatoes. Sprinkle with salt and pepper. Stir lightly until the butter has melted and has been absorbed by the mashed potatoes.
Pour in the milk or cream (and herbs, if using). Again, stir lightly just until the mixture acquires a uniform consistency. Taste, add more salt and pepper, as needed. Stir to blend.
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