Sift together flour, cocoa powder, baking soda, and salt; set aside.
Beat butter and sugar together until light and fluffy, about 2 minutes.
Add egg and vanilla extract to the creamed butter and sugar, mixing until smooth and pale.
Gently stir dry ingredients and espresso powder into the wet mixture until just combined.
Fold in chopped Cadbury mini eggs evenly; cover and refrigerate dough for 1 hour.
Preheat oven to 350°F (177°C).
Portion dough onto baking sheet spaced 3 inches apart and slightly flatten each.
Bake 10-12 minutes until edges are set but centers remain fudgy.
Cool on baking sheet 2-3 minutes, transfer to rack to cool completely.
Sprinkle flaky sea salt on top of cooled cookies before serving.