Preheat your oven to 350°F (175°C). In a medium bowl, sift together flour, cocoa powder, baking soda, salt, espresso powder, and cinnamon.

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

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

Add egg and vanilla extract; beat until smooth.

Gradually mix in dry ingredients until just combined.

Gently fold in chopped dark chocolate and mini eggs.

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

Line a baking sheet with parchment paper.

Drop rounded portions of dough onto the sheet, spaced 2 inches apart.

Sprinkle each dough ball with flaked salt.

Bake for 10 to 12 minutes; remove when edges are set.

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