Preheat the oven to 350℉/180℃. For easy removal of the cake, place parchment paper on the bottom and 2 sides of the pan. Gently grease the two sides without parchment paper and then set aside.
In a medium sized bowl, add in flour, oats, cornstarch, cinnamon, nutmeg, baking powder, baking soda, and salt. Stir to combine and set aside.
1 1/4 cups all-purpose flour, 1 cup rolled oats, 1/2 tsp cinnamon, 1/4 tsp nutmeg, 1 tbsp cornstarch, 1 1/2 tsps baking powder, 1/2 tsp baking soda, 1/2 tsp salt
In a large bowl, add softened butter and white sugar. Beat on medium speed for 2-3 minutes until it’s light and fluffy. Make sure to scrap the sides of the bowl.
1/2 cup butter, 3/4 cups white sugar
Add in the eggs, vanilla, sour cream, eggnog, and rum extract (optional). Continue mixing on medium speed until smooth.
2 eggs, 1 tbsp vanilla, 3/4 cups eggnog, 1/4 tsp rum extract, 1/4 cup full fat sour cream
Mix in the dry ingredients and using a spatula, gently fold until a batter forms. Do not overmix!
Spread the batter into the prepared pan evenly. In a small cup, add white sugar, cinnamon, and nutmeg and mix together. Then sprinkle on top evenly. This will give you a sweet crunchy top.
1 1/2 tbsp white sugar, 1/2 tsp cinnamon, 1/4 tsp nutmeg
Bake for 35-45 minutes, depending on your oven. It took me 45 minutes. Allow it to cool, slice, and enjoy!