Preheat your oven to 375 degrees Fahrenheit and spray a muffin pan with cooking spray. Set aside
In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt.
1 1/2 cups flour, 1/2 cup granulated sugar, 1 1/2 teaspoon baking powder, 1/2 teaspoon baking soda, 1/4 teaspoon table salt
Whisk together the egg and eggnog. Then add the butter. Gently fold the egg/eggnog/butter mixture into the dry ingredients.
1 cup eggnog, 1 egg, 8 tablespoons unsalted butter
Gently fold the egg/eggnog/butter mixture into the dry ingredients.
Spoon into the muffin tin and bake for 20 to 25 minutes or until a toothpick inserted into a muffin in the middle of the batch comes out with just a few crumbs. While the muffins are cooking, make the glaze. Mix together the powdered sugar, rum, cinnamon, and 2 tablespoons of eggnog.
When you bring the muffins out of the oven, let them cool until they won't burn your fingers, dip them in the glaze, and set them on a wire rack.