Prep by preheating the oven to 300° F. Line a cookie sheet with parchment paper or a silicone liner and set aside.
In the bowl of a stand mixer or with an electric hand mixer, cream together the butter, brown sugar and granulated sugar until light in color and fluffy in texture. This should take about 2-3 minutes.
Add in the egg and vanilla, then mix until combined.
Next, add the flour, cornstarch, baking soda and salt. Mix until fully incorporated.
Finally, add in the chocolate chips and Frosted Flakes and mix until evenly distributed.
Using a #40 cookie scoop, portion out the dough into equal 1 1/2 tbsp dough balls and arrange them on your cookie sheet. The cookies will flatten and spread, so I typically do not place more than 12 cookies max on each cookie sheet.
Bake at 300° for 15-17 minutes or until the Milk Bar cookies have flattened and the edges are turning golden brown.
When the cookies are ready, remove them from the oven and allow them to cool on the pan.
Enjoy!