Preheat oven to 400° F (200° C). Line baking sheet with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter until smooth. Add the sugars and beat until light and fluffy. This takes about two to three minutes.
Add the eggs one at a time and mix until smooth. Scrape down the sides of the bowl after mixing each egg.
Add the vanilla and mix for about 20-30 seconds.
With the mixer on low speed, add the all-purpose flour, salt, baking soda, baking powder, and cornstarch. Mix until just combined.
Add the chocolate chips and crushed Cadbury Mini Eggs and mix on low speed. I save some crushed candies to press into the rolled cookie dough balls. After mixing in the chocolate chips and candies, remove the bowl from the mixer and take out the paddle attachment and give the dough a good stir with a silicone spatula.
Using a #24 cookie scoop (about 2.5 ounces), portion dough and place on the baking sheet about two inches apart. Press remaining crushed candies into rolled cookie dough balls. Bake for 9-11 minutes at 400° F.
For large bakery style cookies, divide the dough into 5 ounce balls and place on the baking sheet about three inches apart. Bake for 12-14 minutes.
For mini cookies, use a #40 cookie scoop (about 1.2 ounces) and bake for 7-9 minutes.
Cool on baking sheet for 5-10 minutes. Finish cooling on a cooling rack.