Prepare the Pan: Preheat your oven to 350°F. Grease and flour a 10-cup Bundt pan thoroughly, ensuring all crevices are covered.
Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
2 ½ cups all-purpose flour, 2 ½ teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon salt, 1 teaspoon ground cinnamon, ½ teaspoon ground nutmeg
Combine Wet Ingredients: In a small bowl or measuring cup, stir together the eggnog, sour cream, and rum (if using).
1 cup eggnog, ½ cup sour cream, 2 tablespoons dark rum
Cream Butter and Sugars: In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar on medium-high speed until light and fluffy, about 3-4 minutes.
1 cup unsalted butter, 1 ½ cups granulated sugar, ½ cup light brown sugar
Add Eggs and Vanilla: Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
4 large eggs, 2 teaspoons pure vanilla extract
Combine Wet and Dry Ingredients: With the mixer on low speed, alternate adding the dry ingredients and the eggnog mixture, starting and ending with the dry ingredients. Mix until just combined; do not overmix.
Bake: Pour the batter into the prepared pan and smooth the top. Bake for 40-50 minutes, or until a toothpick inserted into the center comes out clean.
Cool the Cake: Let the cake cool in the pan for 5-10 minutes, then turn it out onto a wire rack to cool completely.
Make the Frosting: In a medium bowl, beat the softened butter on medium-high speed until creamy. Gradually add powdered sugar, mixing on low speed until combined. Add the eggnog, nutmeg, vanilla extract, and salt, then beat until smooth. Adjust with additional eggnog if needed for consistency.
½ cup unsalted butter, 2 cups powdered sugar, 2 to 3 tablespoons eggnog, ¼ teaspoon ground nutmeg, ½ teaspoon pure vanilla extract, Pinch of salt
Frost the Cake: Once the cake has cooled, drizzle or spread the frosting over the top. Slice and serve!