Add the butter, brown sugar and granulated sugar to a large mixing bowl. Using an electric hand mixer or a stand mixer, cream the butter and sugar together for 1-2 minutes, until pale and creamy. *If using a stand mixer, you may need to scrape the sides of the bowl once or twice.
Add the egg and vanilla extract to the bowl and mix on medium speed to combine, scraping the insides of bowl as necessary.
Add the flour and baking soda and beat on a low speed until the ingredients are just combined. Add in the mini eggs and chocolate chips.*Optional: You can break the Cadbury egg candies before mixing into the cookies. Place in a Ziploc bag and use a rolling pin or the flat side of a meat mallet to hit until they crack.
Cover the bowl and chill cookie dough in the fridge for at least 1 hour.
When the dough is cold, preheat the oven to 325°F/160°c. Line 2 baking sheets with parchment paper and set aside.
Scoop the dough using a 1.5 tablespoon cookie scoop and roll portions into balls (They don't have to be perfectly round). Place the dough balls on the prepared baking sheets.
Bake the cookies in preheated oven for 10-12 minutes, or until light golden brown. The cookies will still be soft at this stage, but will firm up as they cool.
Remove from the oven and transfer pans to a wire cooling rack. Allow the cookies to cool completely on the baking sheets.
Notes
Be sure to use Cadbury MINI eggs and not the chocolate Cadbury eggs which are larger and have a creamy center.
Don't skip chilling the dough. If the dough isn't cold, it will spread too much in the oven, causing the Easter cookies to be flat and crispy.
Baked cookies will stay fresh for about 5 days. Store at room temperature in an airtight container. To keep them for up to 3 months, freeze cookies in an airtight container.