Preheat the oven to 350 degrees.
Prepare a 9X13 pan by spraying it with a non stick baking spray.
Melt your butter in a microwave safe bowl for 30 seconds.
In a large bowl, add the butter, brown sugar and granulated sugar. Stir to combine.
Allow the butter mixture to cool for 2-3 minutes.
Add eggs, egg yolk and vanilla extract. Stir to combine.
In a separate medium sized bowl whisk together flour, cornstarch, baking powder and salt.
Add the flour mixture to the butter mixture. Stir to combine.
Add the chocolate chips and M&M's to the mixture. Stir to combine.
Pour the mixture into your prepared pan and spread out evenly.
Bake for 20-25 minutes.
Remove from the oven and carefully place Easter Bunny Peeps on top of the cookie bars.
Place back in the oven for 5-10 minutes.
Remove from the oven and let cool for 5-10 minutes.