Prep the basics: Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper. Lightly crush the mini eggs—aim for halves and big chunks rather than crumbs.
Melt the chocolate and butter: In a heatproof bowl, melt butter and dark chocolate together in short microwave bursts or over a double boiler.
Stir until smooth and glossy. Set aside to cool slightly.
Whisk the sugars and eggs: In a large bowl, whisk granulated sugar, brown sugar, and eggs for about 1 minute until thickened and slightly pale. Mix in vanilla.
Combine with chocolate: Pour the lukewarm chocolate mixture into the egg mixture and whisk until fully blended.
Fold in dry ingredients: Sift in flour, cocoa powder, baking powder, and salt.
Fold gently with a spatula until just combined. The batter should be thick and shiny, like brownie batter.
Add the mix-ins: Stir in most of the crushed mini eggs and any optional chocolate chips, saving some for topping.
Scoop and top: Use a medium cookie scoop (about 2 tablespoons) to portion dough onto the trays, leaving space to spread. Press a few extra mini egg pieces on top of each mound for a pretty finish.
Bake: Bake 9–11 minutes, until the edges are set and the centers look slightly soft and cracked.
Don’t overbake—carryover heat will finish them.
Set and cool: Let cookies rest on the tray 5 minutes, then transfer to a wire rack to cool completely. They’ll firm up as they cool, keeping that fudgy center.