Line a 9x9-inch pan with aluminum foil and spray with nonstick spray.
In a large pot, heat butter, sugar, eggnog, and heavy cream over medium heat, stirring until sugar is dissolved. Bring to a boil and cook, stirring frequently, until a candy thermometer reads 234F (soft ball stage), about 5 minutes.
Remove from heat. Stir in vanilla, white chocolate, and marshmallow cream until smooth. Fold in pecans.
Pour into the prepared pan and rotate pan so fudge reaches the corners. Chill at least 4 hours before cutting into 36 pieces.