Chop the chocolate roughly and set it aside. Next, roughly chop the Cadbury mini eggs and set them aside in a separate bowl. Be careful when chopping the eggs, they are tough, and the knife might slip when you cut them. Take care not to cut yourself.
Dough: Mix the butter, sugar, and vanilla until pale and fluffy. Add egg and combine. Add flour and baking soda and mix with a spoon or a spatula. Add all the chocolate and about half of the chopped mini eggs. Keep some of the larger pieces to press on top of the cookies.
Refrigerate: Wrap the dough and refrigerate for at least a few hours and preferably overnight. (Note 2).
Preheat the oven to 180 degrees Celsius/ 350 degrees Fahrenheit. Line 2 – 3 baking trays with parchment paper, you will have to bake in batches. Keep the balls that note in the oven refrigerated until they go in the oven (Note 3).
Form cookies: Remove the dough from the fridge. The heat of your hands will be enough for the dough to soften enough so that you can roll it properly. Form balls each weighing about 30-35 g/ 1-1.2 oz.
Top: Place on the baking trays leaving space between each one as they will spread. Press some of the reserved chopped Cadbury eggs on each cookie, more in the middle than on the sides.
Bake for about 14 minutes until slightly golden around the edges but still rather pale in the middle.
Cool: Remove from the oven and rest on the baking trays for about 5 minutes. Transfer to wire racks to cool completely.