Cream butter and sugar in a large mixing bowl. Add eggs and beat for about 5 minutes. Beat in eggnog, vanilla, and rum extract.
Whisk together the flour, baking powder, and salt. Stir into the wet ingredients.
Pour HALF of the batter into a well greased and floured bundt pan. (I used Baker's Joy.)
Stir together the nutmeg and cinnamon to remove any lumps. Add to the remaining batter and stir till well combined. Pour over the batter in the pan and swirl with a butter knife.
Bake at 350° for 55-60 minutes or until a toothpick comes out clean. Cool for 15 minutes, then invert onto a cooling rack. Cool completely. Serve with Caramel Eggnog Sauce.
Notes
You can substitute 3 cups cake flour for the 5 Tbsp cornstarch and 2 2/3 cups all purpose flour.