In a large bowl, mix together the butter, sugars until light and fluffy. Then add the egg and vanilla extract until combined, set aside.
In another bowl whisk together the flour, baking powder, baking soda, cornstarch, salt, and rolled oats.
Add the dry ingredients into the wet ingredients and mix until combined.
Then add the rice cereal, chocolate chips, peanut butter chips, and sprinkles and mix.
Cover with plastic wrap and refrigerate at least one hour.
Preheat oven to 350F and bake for 14-16 minutes.
Transfer to cookie rack and allow to cool for about 2 minutes. Serve