In a large mixing bowl, add in the butter and Confectioners' sugar, and mix for about 5 minutes until lightened in color, fluffy, and creamy. If you don't have a mixing bowl, you can add the two into a large bowl, and use a hand mixer to mix them till you achieve the same results.
Add in the pure vanilla extract or any other flavoring, and mix for about 1 minute.
Add the sieved flour and cornstarch to the bowl, and mix till they're well combined.
Line your baking sheet with parchment paper.
Scoop the mixture and roll into 36 similarly sized balls. Place on the lined baking sheet, and press their top with a spoon coated in flour (to prevent sticking), then top with sprinkles.
All the dough to sit in the fridge for about 1-2 hours.
While the dough is sitting, preheat your oven to 300°F (150°C).
Bake the cookies in the oven for about 15 minutes or till their tops look set.
Allow to cool before you start serving.