Preheat your oven to 350 degrees F and spray a 9-inch pie dish with a non-stick spray. Set aside.
In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In the bowl of your stand mixer, with paddle attachment fixed (or with a hand held electric mixer), cream together the butter and sugars until smooth, creamy and fully combined, about 1 minute. Add the egg, milk and vanilla extract. Beat again, until incorporated.
With the mixer set on low speed, gradually add the dry ingredients to the wet. Mix until fully incorporated, about 1 minute. Fold in the white chocolate chips until evenly dispersed.
Pour the cookie dough into the prepared pan. Smooth over so that it is even. Bake at 350 degrees F for 25 minutes or until the top is lightly browned. The center should still be soft to the touch.
Remove from heat and transfer to a wire rack to cool completely before you frost the cookie.
Prepare your frosting by creaming together the butter and confectioner's sugar in the bowl of your stand mixer until fully combined, about 1-2 minutes. Add the milk, vanilla extract and salt and mix again for 1-2 minutes.
Frost the cookie cake once cool, sprinkle with sprinkles, cut into slices and serve!
The sugar cookie cake will store at room temperature in an airtight container for 3-4 days.