Spray 9x13 pan with non-stick cooking spray and set aside.
In a large pot, melt butter and 4 cups marshmallows over medium low heat until marshmallows are melted completely. Remove from heat. Stir in run extract and nutmeg. Pour in cereal and stir to coat in marshmallow. Stir in remaining 2 cups mini marshmallows. Pour into prepared pan and press down with buttered fingers. Cool to room temperature, about 20-30 minutes.
In a small bowl, stir powdered sugar and egg nog together until smooth. Drizzle over bars and sprinkle with more nutmeg. Cut into bars and serve.