Preheat oven to 350 degrees. Line baking sheets with parchment paper or baking mats.
In the bowl of a stand mixer, cream the butter, sugars and vanilla until light and fluffy, about 2-3 minutes. Add the egg and mix until incorporated.
In a separate mixing bowl, whisk together the flour, baking powder, baking powder, cornstarch, salt and oats. With the mixing speed on low, gradually incorporate the dry ingredients. Remove the bowl and fold in the rice cereal and chocolate chips with a spatula. Cover and place dough in the refrigerator for 30 minutes.
Using a large cookie scoop (I like my #20) or a ¼ c. measuring cup, form large balls of dough and place 6 on a sheet at a time. Bake for 14-16 minutes or until lightly golden brown on the tops and crispy on the edges. Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.
Notes
-These cookies are fantastic with chocolate chips or butterscotch chips. I've also made them with toffee in them and I've made a cranberry white chocolate version as well.