Beat butter and icing sugar in stand mixer.
Add vanilla and beat until creamy.
In separate bowl, combine flour, cornstarch, baking powder and salt.
Add half of flour mixture to butter and sugar mixture and beat until combined.
Stir in butterscotch chips and the rest of flour mixture with a wooden spoon.
Roll into 1-inch balls and lightly press down with a floured glass.
Sprinkle very lightly or to taste with sea salt.
Bake at 350°F for 12-15 minutes.