Preheat the oven to 350℉. Spray a 9X9- inch square baking pan with baking nonstick spray. Line parchment paper on the bottom and spray again. Set aside.
In a medium bowl, sift the flour. Add in the cornstarch, baking powder, baking soda, salt, and nutmeg. Set aside.
2 1/2 cups All-purpose flour, 1 TBSP Cornstarch, 1 tsp Baking powder, 1/4 tsp Baking soda, 1/2 tsp Salt, 1/2 tsp Ground nutmeg
Using a mixer in a large bowl, beat the butter, oil, and sugar on high speed for 2 minutes. It will be pale and fluffy.
1/2 cup Unsalted butter, 2 TBSP Oil, 1 cup White granulated sugar
Add in the vanilla, sour cream, and eggs. Mix on medium speed until combined.
1 TBSP Pure vanilla extract, 1/2 cup Sour cream, 3 Large eggs
Add in the flour mixture and eggnog. Mix on low speed until just combined. Take off the mixer. Use a rubber spatula to mix the batter to make sure it is all combined.
3/4 cup Eggnog
Pour cake batter into the prepared pan. Bake for 34-37 minutes. Bake until a toothpick inserted in the middle comes out clean.
Let the cake sit in the hot pan for 10 minutes before transferring to a cooling rack. Cool cake completely before decorating.