Into large bowl, pass potatoes through ricer, or mash very well until smooth. Stir in 4 tablespoons of the butter until melted. Stir in whipping cream, 1/2 cup of the Parmesan cheese, 1 1/2 teaspoons salt, the pepper and nutmeg until combined. Stir in egg yolks, one at a time, until combined. Transfer potato mixture to large decorating bag fitted with large star tip.