Line an 8x8-inch pan with aluminum foil.

In a large saucepan, combine butter, sugar, eggnog, and heavy cream over medium heat; bring to a boil, stirring constantly. Reduce heat and cook until a candy thermometer reaches 238F. Remove from heat; stir in vanilla, white chocolate chips, and marshmallow cream until smooth. Stir in walnuts.

Pour into prepared pan. Chill at least 2 hours before slicing and serving.