Step 1

Arrange a rack in the middle of the oven and heat the oven to 400°F. Line a baking sheet with parchment paper.

Step 2

Peel 2 1/2 pounds Yukon gold potatoes and quarter each potato. Place the potatoes and 1 tablespoon of the kosher salt in a large pot and add enough cold water to cover them by 1 inch. Bring to a boil over high heat, then reduce the heat and simmer until the potatoes are very tender and can be easily pierced with a fork with no resistance, 14 to 16 minutes. Meanwhile, finely grate 2 ounces Gruyère cheese (about 1/2 cup) and mince 3 garlic cloves.

Step 3

When the potatoes are ready, drain through a colander. Place the empty pot back over medium heat and add 3 tablespoons of the unsalted butter. When the butter is melted, add the garlic and cook, stirring often, until fragrant, 30 seconds to 1 minute. Add 3 tablespoons milk, the remaining 3/4 teaspoon kosher salt, and 3/4 teaspoon black pepper and bring to a simmer. Turn off the heat.

Step 4

Add the cooked potatoes and mash with a potato masher until smooth. (Alternatively, pass the potatoes through a ricer or food mill into the milk mixture.) Add the Gruyère and stir until melted. Add 4 large egg yolks and vigorously stir until they are fully incorporated.

Step 5

Transfer the potatoes into a large piping bag fitted with a star tip or a large zip-top bag with the corner cut off. Pipe the potatoes into 12 (2-inch high, 2-inch wide) mounds on the prepared baking sheet, spacing them at least 1-inch apart. If the bag is too hot to handle, wrap a kitchen towel around the piping bag.

Step 6

Place the remaining 3 tablespoons unsalted butter in a small microwave-safe bowl and microwave in 10-second bursts until completely melted. (Alternatively, melt on the stove.) Brush all of the butter over the tops of the potatoes.

Step 7

Bake until the potatoes are golden brown, 15 to 18 minutes. Meanwhile, finely chop until you have 1 tablespoon fresh chives. Sprinkle the chives over the potatoes and serve immediately.