Gather all ingredients.

Place potatoes in a large pot and cover with cold water to a depth of 2 inches; add 2 teaspoons salt.

Bring to a boil over medium-high heat, reduce heat to medium and simmer potatoes until they are tender and easily mashed with a fork, about 20 minutes.

Preheat the oven to 425 degrees F (220 degrees C).

Drain and return potatoes to the pot. Cook over medium, stirring constantly, for 2 minutes to help dry potatoes.

Mash potatoes and 8 tablespoons butter until mostly smooth.

Add boursin, cream, chives, pepper, nutmeg, and remaining 1 1/2 teaspoons salt; mash until well combined, stopping to scrape down sides as needed.

Add in egg yolks, 1 at a time, stirring between each addition until incorporated.

Spread potato mixture evenly in a 2- to 2 1/4-quart baking dish.

Gently brush the top of potato mixture with remaining 1 tablespoon butter.

Score top as desired using the tines of a fork; wipe fork clean as needed.

Bake in the preheated oven until golden brown, slightly puffed, and an instant read thermometer inserted in center registers 160 degrees F (71 degrees C), about 30 minutes.

Let stand 10 minutes before serving.