Beat butter, sugar and brown sugar using an electric mixer on medium speed for about 5 minutes.
Add egg and vanilla and beat again until combined.
Add the cocoa, flour, baking soda and salt and use the mixer to combine, start on low and increase speed to medium once most of the loose powder is incorporated. It will take 1-2 minutes of mixing for the dough to come together.
Fold in the chocolate chips and 1 cup chopped mini eggs. Reserve the rest for topping cookies before baking.
Use a cookie scoop to scoop 1.5 tbsp of dough per cookie and place on parchment lined baking sheet at least 2 inches apart.
Top each cookie with additional mini egg pieces then bake for 9-10 minutes.
Top with sea salt immediately after removing from the oven.