Preheat your oven to 350 degrees Fahrenheit. Line two 12-mold muffin tins with paper liners; set aside.
In a large mixing bowl, combine all dry ingredients: flour, cornstarch, granulated sugar, baking powder, and salt. Whisk until evenly mixed.
Add the softened butter and vanilla. Beat with a handheld electric mixer until the mixture resembles a coarse meal.
Add the eggs one at a time, beating well after each addition until smooth.
Pour in the eggnog and sour cream, mixing until well combined. Be careful not to overmix; a few lumps are fine.
Fold in the holiday jimmies gently using a rubber spatula.
Divide the batter evenly among the prepared muffin tins, filling each cup about halfway.
Bake for 18 to 20 minutes, or until a cake tester comes out clean. Cool completely before frosting.
Beat the remaining butter in a stand mixer until smooth, gradually adding in the confectioners' sugar until smooth.
Slowly add in the eggnog and vanilla extract, beating until slightly stiff. Refrigerate for 10 minutes.
Frost cooled cupcakes and top with holiday nonpareils.
