Place potato chunks in a large pot and cover with cold water by 1 inch. Add 1 teaspoon kosher salt. Bring to a boil, then simmer until potatoes are fork-tender, about 15–20 minutes.

Drain thoroughly and return potatoes to the hot pot for 1–2 minutes to evaporate excess moisture.

Pass the potatoes through a ricer or mash until smooth.

Fold in warm butter, then whisk in the warmed milk or cream.

Stir vigorously to create a silky texture. Temper and add the 3 egg yolks, mixing quickly so they incorporate without curdling.

Season with 1/2 teaspoon salt and pepper. The mixture should be thick enough to hold a piped shape.

Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper or a silicone mat.

Fit a large star piping tip into a piping bag or use a heavy-duty freezer bag with the corner snipped.

Spoon potato mixture into the bag and pipe 2-inch mounds onto the prepared sheet, leaving space between each.

For a classic look, use a star tip to create ridged edges that crisp up nicely.

If using Parmesan, lightly sprinkle each mound with the grated cheese now.

Brush each mound with the beaten egg wash to encourage golden color.

Bake at 425°F (220°C) until the edges are deeply golden and crisp, about 18–25 minutes.

Visual cue: edges should be golden brown and firm when tapped; centers remain creamy inside.

For an extra-crispy finish, broil on high for 1–3 minutes, watching closely to avoid burning.

Remove from oven and let rest for 2 minutes before transferring to a serving platter.

Garnish with chopped chives.