Using a stand or hand mixer on medium speed, beat together butter and sugars until light and fluffy, about 3 minutes. Add the egg and vanilla and continue beating until combined.
Turn the mixer to low and slowly add flour, cornstarch, baking soda and salt until combined. (You can mix the dry ingredients in a bowl first, but I just add them directly to the mixing bowl to save dishes.)
Add the chopped Easter eggs and mix until just combined.
Scoop heaping tablespoons onto a nonstick or parchment-lined baking sheet, about an inch and a half apart.
Bake 9-10 minutes until bottom edges are JUST barely golden.
Remove from oven and let cool on the pan a few minutes before transferring to a wire rack.
Recipe by Flying on Jess Fuel at https://www.jessfuel.com/milk-chocolate-easter-egg-cookies/