Preheat the oven to 350F degrees. Line baking sheets with parchment paper or baking mats.
In a large bowl beat together the butter, shortening, white sugar and brown sugar until fluffy (about 2 minutes).
Beat in the vanilla and eggs.
In a medium bowl whisk together the flour, cornstarch, baking soda & salt.
Carefully beat the flour mixture into the butter about ½ at a time. I usually start with the mixer on low speed, then turn it up to medium as I go.
Turn off the mixer and stir in 1 ½ cups white chocolate chips.
Form the dough into balls about 1 to 1.5 tablespoons in size. Place 2 inches apart on the lined cookie sheets. Do not flatten.
Bake one tray at a time on the middle rack of the oven for 9-11 minutes or until the tops look just set.
Remove from the oven and dot the tops of each cookie with a few extra chocolate chips (optional).
Cool on the cookie tray for 10 minutes, then transfer to a wire rack to continue cooling.
Notes
Store cookies in an airtight container for up to 5 days.For large cookies: After making the dough, cover the bowl and refrigerate for at least 30 minutes. Form the dough into balls about 3 tablespoons in size ensuring that they're taller than they are wide. Place 3 inches apart on the lined cookie sheets and bake for 13-16 minutes or until the tops look just set. **How to Freeze Cookie Dough. Make the cookie dough and form it into balls. Place the balls in a ziploc bag and freeze for up to 2 months. Bake from frozen - they'll likely take about 1 extra minute to bake. Find out more tips for freezing cookie dough here.