Preheat oven to 350 degrees and line an 8x8 baking pan with aluminum foil or parchment paper. You can also spray your pan with nonstick cooking spray, but using parchment paper or aluminum foil allows for easier removal from pan once baked.
In a mixing bowl, cream together the butter, brown sugar, and white sugar until creamy.
Add in the egg and vanilla, mixing until just combined.
Slowly add in the flour, cornstarch, baking soda, and salt stirring until just combined.
Fold in the chocolate chips and mini eggs.
Press cookie dough into prepared pan and add additional chocolate chips and mini eggs to the top, pressing gently until they sink slightly into the dough.
Place baking pan into the oven for about 18-25 minutes, or until the edges are lightly golden brown. You don't want to overbake these as they will get hard, so try to remove just before cooked through as they continue to bake for a minute or two once removed from oven but still in pan.
Once cooled, cut into desired size squares and enjoy!