Preheat oven to 350 degrees. In a mixing bowl, whisk together flour, cornstarch, baking powder, baking soda and salt, set aside. In the bowl of an electric mixer, whip together butter, granulated sugar, molasses and coffee creamer powder until mixture becomes pale and fluffy, about 4 minutes. Mix in egg and vanilla bean paste.
Slowly add in dry ingredients and mix just until combine. Stir in milk chocolate chips and semi-sweet chocolate chips (if you want it to look fancier, I set aside 2 Tbsp of the chocolate chips and after spreading the dough into the skillet, I sprinkled the 2 Tbsp that were set aside over the top then gently pressed them into the dough. I don't like when the chocolate chips don't show through enough).
Butter a 10 inch cast-iron skillet (I just buttered by rubbing the pan with a chilled stick of butter). Spread cookie dough into an even layer in the skillet. Bake in preheated oven for 25-30 minutes until golden (note: the cookie will continue to cook slightly once removed from oven so judge accordingly).
Allow to cool for about 20 minutes before cutting into slices. Alternately, allow to cool for about 5 minutes and dive into it warm right from the skillet, topped with ice cream drizzled with hot fudge and caramel sauce (that's what I did, warm with cookie dough ice cream, hot fudge and caramel sauce, inexpressibly delicious!).
*if you don't have molasses, alternately you can use 1/2 cup packed light-brown sugar in place of 1/2 cup of the granulated sugar. So, you would be using 1/2 cup light brown sugar and 1/4 cup plus 2 Tbsp granulated sugar.