Preheat oven to 400 degrees F.
In large mixing bowl (best to use a stand mixer fitted uwith the paddle attachment), cream together cold butter, brown sugar, and sugar for a few minutes until it is creamy. About 3-4 minutes.
Add eggs, one at a time, and mixing well. Add vanilla
Stir in flours, cornstarch, baking soda, and salt. Mix just until combined. Add both chocolates.
Take about 1/2 cup dough and place onto tray. Makes about 8 LARGE cookies.
Bake for 9-12 minutes or until golden brown on the top. Let them rest for at least 10 minutes. Transfer to cooling rack.
