FOR THE COOKIE BARS: Preheat oven to 350 degrees. In a mixing bowl whisk together flour, cornstarch, cinnamon, baking powder, salt and powdered milk, set aside.
In the bowl of an electric stand mixer fitted with the paddle attachment, whip together butter, brown sugar and granulated sugar until pale and fluffy, scraping down sides and bottom of bowl occasionally.
Add in eggs one at time mixing until combined after each addition. Blend in sour cream and vanilla. With mixer set on low speed, slowly add in dry ingredients and mix just until combined.
Butter and 13 by 9-inch baking dish, then with buttered hands gently press and spread cookie dough into an even layer in dish.
Bake in preheated oven until toothpick inserted into center comes out clean, 17 - 21 minutes. Allow to cool completely then frost with cream cheese frosting. Cut into squares and store in an airtight container.
FOR THE FROSTING: In the bowl of an electric stand mixer fitted with the paddle attachment, whip together cream cheese and butter until pale and fluffy, occasionally scraping down sides and bottom of bowl.
Stir in vanilla. With mixer set on low speed, slowly add in powdered sugar and continue to mix until fluffy.