How to Make It
Preheat oven to 350°F. Coat 6 (4 ounce) ramekins with butter, and then sugar. Set aside.
Grate frozen butter into flour, and stir to combine.
Combine sugar, milk, cinnamon, and nutmeg in a small sauce pan over high; bring to a boil, stirring often. Reduce heat to medium-low. Stir in rum, and gradually whisk in flour mixture; cook, whisking constantly, until mixture thickens and bubbles, about 5 minutes. Let cool to room temperature, about 15 minutes.
Add egg yolks, 1 at at a time, whisking to combine after each addition. Whisk in vanilla; set aside.
Beat egg whites with an electric mixer on medium-high until soft peaks form; fold into eggnog mixture. Spoon mixture into prepared ramekins, and bake in preheated oven until doubled in size, 20 to 25 minutes.