In a medium bowl, whisk together the flour, baking soda, salt, and espresso powder. Set aside.
In a large bowl or stand mixer, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy (about 3-4 minutes).
Beat in the eggs one at a time, then add the vanilla extract. Mix until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined.
Fold in the chocolate chips and chopped dark chocolate (if using) with a spatula.
Cover the dough and refrigerate for at least 1 hour (or up to 24 hours for even better flavor).
Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
Scoop the chilled dough into balls (about 2 tablespoons each) and place them on the prepared baking sheets, leaving about 2 inches of space between each cookie.
Bake for 10-12 minutes, or until the edges are golden brown but the centers still look slightly underbaked.
Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.