Sift together the flour, cocoa powder, baking soda, espresso powder, salt, and cinnamon in a medium bowl, breaking up any lumps in the cocoa. Chop the dark chocolate into small 1/4-inch chunks and set aside with the mini eggs. This prep work ensures your dry ingredients are evenly distributed and your chocolate mixes uniformly into the dough later.

In a large mixing bowl, beat together the softened butter, packed brown sugar, and sugar for about 1 minute until the mixture is light and fluffy. This creaming process incorporates air into the dough, which helps create a tender, cake-like cookie texture. The packed brown sugar adds moisture and chewiness to the final cookies.

Add the egg and vanilla extract to the butter mixture and beat until fully combined and smooth, about 30 seconds. The egg acts as a binder and adds richness to the dough.

Add the dry ingredient mixture from Step 1 to the wet ingredients and mix on low speed for about 30 seconds, just until the flour is incorporated and no streaks remain. Overmixing develops gluten, which can make cookies tough, so stop as soon as the dry ingredients disappear.

Gently fold in the chopped dark chocolate and mini eggs using a spatula or wooden spoon, distributing them evenly throughout the dough. Cover the bowl and refrigerate the dough for 30-45 minutes. Chilling prevents the cookies from spreading too much during baking and allows the flavors to meld together. I find that the espresso powder really deepens the chocolate flavor after the dough has rested.

Preheat your oven to 350°F and line a baking sheet with parchment paper. Using a cookie scoop or spoon, drop rounded portions of dough onto the prepared sheet, spacing them about 2 inches apart. Sprinkle a tiny pinch of flaked salt on top of each cookie before baking. Bake for 10-12 minutes, until the edges are set but the centers still look slightly underdone and soft.

Remove the cookies from the oven and let them rest on the baking sheet for 15 minutes before transferring to a wire rack to cool completely. This resting period allows the cookies to firm up slightly while still maintaining their fudgy, brownie-like texture in the center. The salt will stay perfectly crunchy and complement the rich chocolate and sweet mini eggs.