Pour the cooled butter into the bowl of your stand mixer (or a large bowl for a hand-held mixer).
Add both the white and brown, salt and vanilla, and mix on medium speed until the butter and sugar mixture becomes creamy.
Add the eggs, and mix until fully incorporated.
In a separate bowl, sift the flour, cocoa powder, baking soda and baking powder together.
Add the cocoa-flour mix in two additions while the mixer is on the lowest speed.
To crush the mini eggs, I found that the best way is to place them in ziploc bag or equivalent and use the rolling pin to crush them. For these cookies, you don't want the pieces to be too small.
When most of the flour is mixed in, add the chocolate chips and mini egg pieces to the bowl and mix on low speed for just a few seconds.
Using a cookie scoop, portion the cookie dough on a parchment paper-lined pans.
Cover the pan and refrigerate for at least one hour. I often leave them in the refrigerator overnight.
If you want to freeze all of some of the dough, you can put the dough in a sealed container and freeze it at this point.
Preheat the oven to 350° F.
Line sheet pan with parchment paper. Place 4 - 6 cookie dough portions evenly spaced apart.
Bake in preheated oven for 10 to 12 minutes. Be careful not to overbake.
Remove the cookies from the oven and let them cool for 5 minutes. Then carefully place the cookies on a wire rack and let them cool down more.
The cookies are best eaten a little warm, but can be enjoyed at room temperature too.