• Spray a 13 x 9 x 2-inch pan with cooking spray. Also spray a large pot and a rubber scraper with nonstick cooking spray. Set aside.
• In the large pot, melt butter and 4 cups marshmallows over medium low heat until marshmallows are melted completely.
• Remove from heat. Stir in rum extract and ground nutmeg.
• Moving quickly, pour in cereal and stir to coat in melted marshmallows. Once mostly coated, stir in the remaining 5 oz marshmallows. They will melt slightly from the heat, but not completely which is what you want.
• Pour into prepared pan and press down with buttered fingers. Cool to room temperature, about 20-30 minutes.

for the optional glaze-

• In a small bowl, stir powdered sugar and eggnog together until smooth.
• Drizzle over bars and sprinkle with more ground nutmeg. Cut into bars and serve.