Preheat oven to 350F degrees. Line an 8 x 8 or 9 x 9 baking pan with parchment.
Beat together softened butter, brown sugar, and sugar for about 3 minutes or until light and fluffy. This is best with an electric mixer, like a Kitchenaid but you can also mix by hand, just make sure all of the sugars are dissolved into the butter.
Add egg and vanilla extract and mix until combined.
With mixer on low, add in flour, cornstarch, baking soda, and salt. Let mix until just combined, but don't over mix.
Fold in milk chocolate chips and mini eggs until well combined.
Press cookie dough into the pan. Dough will be thick so you'll need to use a spatula to press the dough fully into the pan. You can also add extra Cadbury mini eggs into the top of the cookie dough before baking.
Bake for 18-22 minutes or until the edges start to turn a light golden color. These are meant to be chewy in the center so it is better to under bake rather than over bake. The cookies will continue to set up after being removed from the oven, so try to resist cutting into them until they've cooled.
Cut into squares and enjoy!