Preheat oven to 325F. Line 2 baking sheets with parchment paper. Set aside.
Whisk flour and cornstarch together in a medium bowl. Set aside.
Place the butter, icing sugar and vanilla extract in a bowl of an electric mixer fitted with the paddle attachment. Beat until light and fluffy.
Mix the dry ingredients in until just combined. Roll dough into 1 inch round balls. Place them 1 inch apart on the prepared baking sheet. Use a fork to lightly flatten the dough. Decorate with sprinkles.
Place baking sheets into the refrigerator for 10 minutes to firm cookies.
Bake for 15 minutes or until lightly golden. Let cool and store in airtight container for 1 week.
Use real butter!
Chill the cookies before baking them. The butter will firm up and the cookies won't spread out when baking.
Beat the butter and icing sugar for a solid 5 minutes. The ingredients need to be whipped really well to make a light airy cookie.
Only bake 1 cookie sheet at a time. If you bake with 1 cookie sheet on the middle rack, and 1 on the lower rack, the cookies on the bottom can get dark on the bottom.