Place the peeled and cubed potatoes in a large saucepan and add the water and 1 tablespoon salt. Bring to a vigorous simmer over high heat, then reduce to medium and simmer until the potatoes are very tender when pierced with a fork, 15–20 minutes depending on cube size.

Drain and dry

Drain the potatoes thoroughly in a colander. Return the drained potatoes to the hot pot and place the pot back on the turned-off burner for 1–2 minutes to evaporate excess moisture, shaking the pot occasionally to prevent sticking.

Mash and incorporate fat

Using a potato ricer or masher, press or mash the potatoes until smooth and free of lumps. Immediately add the butter and stir until melted and incorporated.

Adjust texture

Gradually add the warmed milk (and cream if using) a few tablespoons at a time, stirring gently after each addition, until the mash reaches a creamy, smooth consistency. Be careful not to overwork the potatoes to avoid a gluey texture.

Add cheese and season

Off the heat, fold in 120–150 g of the grated mature cheddar until melted and evenly distributed. Taste and season with up to 1 teaspoon salt and 1/2 teaspoon freshly ground black pepper, adjusting to your preference.

Finish and serve

Transfer the cheesy mash to a serving dish. Sprinkle the remaining 30 g of grated cheddar (or more if desired) over the top so it melts from residual heat. Garnish with chopped chives if using and serve immediately.