In a stand mixer or large mixing bowl, beat softened butter until smooth. Add sugars and beat on medium-high until fluffy. Add starter, eggs, eggnog, and rum extract. Set aside.
In a medium bowl, whisk together flour, pudding mix, baking powder, baking soda, nutmeg, cinnamon, and salt.
Add flour mixture to wet mixture, stirring until fully incorporated (be careful not to overmix). Batter will be thick.
Pour batter into Bundt pan until ⅔ full, making sure batter touches the sides. Smooth the top with a spatula.
Bake for 45 minutes or until a toothpick inserted in the center of the cake comes out clean. If the top of your cake begins to brown too quickly, tent foil on top for the last 10-15 minutes.
Let cake rest in pan for 5-10 minutes, then turn upside down onto a cooling rack. After 10 minutes, remove the pan from the cake, and allow it to cool completely.
ENJOY!
Notes
Want a richer flavor? Substitute vanilla instant pudding with cheesecake instant pudding.
Make this eggnog cake kid-friendly by replacing rum extract with vanilla extract.
Since Bundt pans vary in shape, use any leftover batter for single cupcakes (I got two from mine). I dropped the heat to 325° F (165° C) and baked for 25 minutes.