Preheat your oven to 350°F (175°C). In a sizable mixing bowl, beat the butter and sugar together until the mixture becomes light and fluffy, approximately 5 to 7 minutes. Add the eggs, one at a time, followed by the vanilla extract, and mix thoroughly.

In a separate bowl, whisk together the flour, cocoa powder, espresso powder, baking soda, and sea salt. Gradually incorporate the dry ingredients into the creamed mixture until fully combined. Gently fold in the chopped dark chocolate.

Drop tablespoon-sized portions of dough onto ungreased baking sheets, spacing each portion about 2 inches apart. Sprinkle a light amount of additional flaked sea salt on each cookie. Bake in the preheated oven for 10 to 12 minutes or until the edges are set. Remove from the oven and transfer the cookies to a wire rack to cool completely.