Add vanilla and powdered sugar and beat until well mixed.
In a small bowl, whisk together flour, cornstarch, baking powder and salt until there are no lumps.
Mix dry ingredients into butter mixture until thoroughly combined.
If the mixture feels soft, chill for at least a half hour. Otherwise, go ahead and proceed to baking.
Roll dough into 1″ balls (I like to use my small cookie scoop to portion the dough.)
Place on ungreased cookie sheet and flatten slightly.
Bake for about 12 minutes, or until lightly golden brown.
Cool on cookie sheet for 2 minutes, then remove to wire rack to cool completely.
To glaze: mix together powdered sugar, vanilla. Add milk or cream a little bit at a time until you form a thick glaze consistency. Spoon a small amount onto each cookie and top with sprinkles if desired.