Preheat oven to 350°F (175°C).
Thoroughly grease and lightly flour a 10–12 cup Bundt pan.
Whisk flour, baking powder, baking soda, nutmeg, cinnamon, and salt.
Beat softened butter and sugar for 3–4 minutes until light and fluffy.
Add eggs one at a time, mixing after each addition.
Mix in vanilla and rum extract.
Whisk eggnog and sour cream together separately.
Alternately add the dry ingredients and eggnog mixture to the butter mixture, beginning and ending with dry ingredients.
Mix only until combined.
Gently finish folding the batter with a rubber spatula.
Spoon batter evenly into the prepared Bundt pan.
Smooth the surface and gently tap the pan once or twice.
Bake for 45–55 minutes, until a skewer inserted into a thick section comes out clean or with a few moist crumbs.
Cool in the pan for approximately 15 minutes.
Carefully invert onto a cooling rack and cool completely.
Whisk powdered sugar, 2 tablespoons eggnog, rum, vanilla, and nutmeg together.
Add additional eggnog by the teaspoon until the glaze is thick but pourable.
Drizzle over the completely cooled cake.
Allow the glaze to set for 15–20 minutes before slicing.