Preheat the oven to 400ºF. Line two baking sheets with parchment paper or silicone baking mats and set aside
In the bowl of a stand mixer with the paddle attachment affixed, add the cubed butter, brown sugar and granulated sugar
Turn on the mixer to low for a few seconds, then gradually increase speed to medium-high, mixing for about 3-4 minutes until well incorporated and fluffy. The mixture will first gather around the paddle then eventually begin sticking to the sides of the bowl. This is a good sign it’s done mixing
Stop the mixer and scrape the sides and bottom of the bowl. Turn the mixer to low and add egg and egg yolk then vanilla. Mix on medium for about 30 seconds until well incorporated, stopping to scrape down the sides and bottom with a rubber spatula, as needed.
With the mixer off, add in the flour, salt, baking powder and baking soda. Turn the mixer on low and stir until just combined. Do not over mix.
Use the end of a rolling pin or a wooden mallet to crush the mini eggs. This is best done in the sealed bag they came in or in a sealed ziplock bag, with as much air as possible pressed out of the bag. You want some pieces bigger and some whole pieces
To the cookie dough, add in 3⁄4 of the crushed mini eggs, reserving the remainder to press into the tops of the cookie dough just before baking. Then add in the chocolate chips. Turn the mixer on low and stir just until combined. Turn the mixer off and, using your hand, mix or knead the dough gently to ensure mini eggs and chocolate chips are evenly incorporated
Portion dough into 2 ounce size balls, rolling between your palms to ensure a smooth ball. Place about 2 inches apart on prepared baking sheets. Gently press reserved mini eggs into the tops of the cookie dough balls
Bake in the preheated oven for 5-6 minutes then rotate baking trays and bake for another 4-5 minutes until the tops of the cookies are just starting to brown and the middle no longer looks wet. Do not overbake. If the cookies spread too much, place a large round cookie cutter or overturned small bowl around the hot cookies and move the cookie cutter around in a circular motion. This will help bring the edges of the cookies in, creating rounder cookies. This step is optional
Allow the cookies to cool for 10 minutes on the hot trays then transfer to wire racks to cool