In a bowl of a stand mixer fitted with a paddle attachment, cream butter, and brown and granulated sugar together on medium speed, beating until light and fluffy about 3 minutes. Add in egg and vanilla and mix until incorporated on low speed. Turn the mixer up to medium-high and mix for 1 minute.

Scrape down the sides of the bowl if needed. Add the cornstarch, baking soda, and salt. Mix for 30 seconds then add the flour 1 cup at a time. Mix until fully incorporated.

Add the chocolate chips and crushed Cadbury mini eggs and either beat for a few seconds on low speed or fold in by hand.

Drop 2 tablespoons of the dough or roll into balls onto a lined baking sheet. Top with the slightly crushed Cadbury mini eggs and more chocolate chips. Cover with plastic wrap and chill the dough for at least 1 hour or longer. (Overnight or a couple of days works too.) Or chill for 20-25 minutes in the freezer.

When ready to bake, preheat oven to 350 F. Spread cookie dough on a baking sheet lined with parchment paper, spaced 2 inches apart. Bake for 8-10 minutes until the edges are just golden. Make sure not to over bake, the cookies will set up as they cool.

Remove from oven, let cool on baking sheet for 5-10 minutes, and then transfer to wire rack to finish cooling. (Store in an airtight container.)