In a heavy saucepan, combine butter, eggnog and sugar. Bring to a full rolling boil over medium-high heat, stirring constantly. Reduce heat to medium, and continue to boil, stirring frequently, 8 to 10 minutes or until mixture reaches 234°F (use candy thermometer).
Remove from heat; add chocolate and nutmeg; stir until smooth. Add marshmallow creme, pecans and rum; mix well. Pour mixture into a 9-inch square pan lined with buttered foil. Cool completely; cut into 1 1/2-inch squares.
Store in an airtight container in the refrigerator for up to one week.
In a 4-quart microwave-safe bowl, microwave butter on high for 1 minute or until melted. Add eggnog and sugar; mix well and microwave 6 minutes or until mixture comes to a rolling boil; stirring after 3 minutes.
Scrape sides of bowl and mix well, continue microwaving 9 minutes more, scraping bowl and mixing after each 3 minutes, stir in chocolate until melted; continue as directed above.