In a large bowl, sift together the flour, cornstarch, baking powder, baking soda and salt. Then whisk together.
In a separate large bowl, beat the butter brown sugar and granulated sugar until combined and no lumps of butter remain. This is definitely easier if you have a stand mixer.
Beat the eggs and vanilla into the butter mixture. Be sure to turn off the mixer and scrape down the sides and bottom of the bowl a few times.
Mix the flour mixture into the butter mixture. I typically do this about 1/2 at a time, starting with the mixer on a low speed. The dough will be thick. After all the flour is incorporated, squeeze the dough between your fingers. If it sticks to your fingers, mix in 1 more tablespoon of flour
Chop 1/2 cup of Mini Eggs. Mix the chopped Mini Eggs, full Mini Eggs and chocolate chips into the cookies.
Cover the bowl and place the dough in the fridge to chill for at least 1 hour or up to 48 hours.
When ready to bake, preheat the oven to 375℉ (190℃). Line cookie sheets with parchment paper or silicone baking mats.
Form the dough into balls with about ¼ to ⅓ cup of dough each - I use a scale to make uniform dough balls and each weighed about 3½ ounces or 100 grams. I got 14 cookies in total. Place the dough balls at least 2½ inches apart on the lined cookie sheets.
Bake 1 cookie sheet at a time in the middle of the preheated oven. Cookies will take about 12-14 minutes to bake, or until the tops look just set.
Remove from the oven. Cool the cookies on the cookie tray. For perfectly round cookies, when the cookies come out of the oven take a large biscuit cutter, round cookie cutter or large cup that is bigger than the cookies. Place around each warm cookie and gently make a circular motion to mold each cookie into a perfectly round shape.