Preheat your oven to 350°F (180°C) and line a baking sheet with parchment paper.
Measure out the crushed Cadbury Mini Eggs and mini chocolate chips, then set them aside.
In the bowl of a stand mixer, beat the butter, brown sugar, and granulated sugar together until the mixture is smooth, light, and fluffy—this will take a few minutes.
Scrape down the sides of the bowl, then add the egg and vanilla extract. Mix until fully incorporated.
Once everything is well combined, stop the mixer and add in the flour, rolled oats, baking soda, and salt.
Begin mixing on low speed. When the flour mixture is about halfway incorporated into the dough, add in the crushed Cadbury Mini Eggs and mini chocolate chips. Continue mixing until everything is evenly distributed.
Stop mixing as soon as all the flour is incorporated—be careful not to overmix the dough.
Portion the dough into 1/3 cup mounds. Shape each portion loosely into a tall dough ball, leaving the tops rough and jagged for extra texture. Avoid pressing the dough together too firmly.
Repeat the process until all the dough is portioned.
Place the dough balls onto the prepared baking sheet and bake for 13–15 minutes, or until the cookies have spread and the edges begin to turn golden brown.
Right after removing the cookies from the oven, use the back of a metal spoon to gently nudge the edges of each cookie toward the center to create perfectly round shapes.
Let the cookies cool directly on the baking sheet for 20 minutes to set.
Serve them warm and enjoy!