Sift the flour, cocoa and baking soda together.
Working with a stand mixer fitted with the paddle attachment, beat the butter and both sugars together on medium speed until soft and creamy, about 3 minutes. Add the salt and vanilla. Turn off the mixer, add all the dry ingredients and pulse a few times to start the blending. Turn the mixer to low and beat until the dough crumbly texture.
Turn the dough out onto a work surface and gather it together, kneading it if necessary to bring it together. Divide the dough in half and then shape the dough into logs that are 11/2 inches in diameter. Wrap the logs in plastic wrap and freeze them for at least 2 hours or refrigerate them for at least 3 hours.
When you’re ready to bake: Center a rack in the oven and preheat it to 325 degrees F. Line two baking sheets with parchment paper.
Working with one log at a time and using a long, sharp knife, slice the dough into 1/2-inch-thick rounds. (The rounds might break as you’re cutting them — just squeeze the bits back onto each cookie.)
Dip each cookie into the crushed mini eggs and arrange the rounds on the baking sheets, leaving about 2 inches between them.
Bake the cookies for 12 minutes. Transfer the baking sheet to a cooling rack and let the cookies rest until they are only just warm.