Special equipment: a piping bag fitted with a star tip
In a large pot, combine the potatoes, 1 teaspoon salt and enough cold water to cover. Bring to a boil over high heat. Lower the heat and simmer until fork tender, 10 to 12 minutes. Drain the potatoes and then press through a ricer or food mill back into the pot they were cooked in. Place over low heat for a few minutes to dry the potatoes.
Heat the cream and 1/2 stick of the butter in a saucepan over medium-low heat until the butter melts and the cream is warm but not hot. Stir the mixture into the potatoes. Mix in the egg yolks and season with salt and pepper. Add cheese and nutmeg if desired.
Preheat the oven to 400 degrees F. Grease a medium baking dish with butter.
Melt the remaining 1/2 stick butter in a small pot over low heat.
Fill a piping bag fitted with a star tip with the mashed potatoes. Into the prepared baking dish, pipe the potatoes in a circular motion, about 3 inches in diameter, starting from the outside and filling in a circle. Finish by piping upwards for a nice swirl on top. Repeat to fill the entire baking dish. Brush each circle with the melted butter. Bake in the oven until golden brown, 30 to 40 minutes.