Spray a 9x13 glass baking dish with cooking spray and set aside.
In a stand mixer or with a hand mixer, cream together the butter, brown sugar, and granulated sugar for 1 minute.
Next, add the egg, egg yolk, and vanilla. Mix again until completely combined.
Add flour, cornstarch, baking soda, and salt. Mix well. (If desired, you can mix these ingredients together in a separate bowl before adding to the creamed mixture.)
Once you have the dry ingredients mixed in, add the Cadbury Mini Eggs. I prefer to chop mine in half before adding, but you can leave them whole if you prefer. (Leave a few out of the mix, so you can use them to make the top of the bars look pretty.)
Once the mini eggs are mixed in, evenly press the dough into the prepared 9x13 pan and bake at 350 degrees for 22-28 minutes. Mine needed 27 minutes.
The bars should be slightly browning when they are done.
Let cool completely before cutting.
Enjoy!
Recipe by Real Life Dinner at https://reallifedinner.com/cadbury-mini-egg-blondies/