Sift together the all-purpose flour, cocoa powder, baking soda, espresso powder, salt, and ground cinnamon into a medium bowl and set aside.

Chop the dark chocolate into 1/4-inch chunks and set aside along with the mini eggs.

In a large mixing bowl, beat together softened unsalted butter, packed brown sugar, and granulated sugar until fluffy and light.

Add the egg and vanilla extract to the creamed mixture and mix until smooth.

Slowly incorporate the sifted dry ingredients into the wet mixture until just combined.

Gently fold in the chopped dark chocolate and mini eggs, being careful not to crush the eggs.

Cover the dough with plastic wrap and refrigerate for 30 to 45 minutes.

Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

Drop rounded portions of the dough onto the prepared baking sheet, spacing them about 2 inches apart. Sprinkle flaked salt on top.

Bake in the preheated oven for 10 to 12 minutes, until edges are set but centers look underdone.

Let the cookies cool on the baking sheet for 15 minutes before transferring to a wire rack to cool completely.