Step 1: Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats.
Step 2: In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until well combined. The mixture should be smooth.
Step 3: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 4: In a separate medium bowl, whisk together the flour, cocoa powder, espresso powder (if using), baking soda, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined. Be careful not to overmix.
Step 6: Fold in the chocolate chips or chunks until evenly distributed throughout the dough.
Step 7: Cover the bowl with plastic wrap and chill the dough in the refrigerator for at least 30 minutes to 1 hour. This helps prevent spreading and enhances chewiness.
Step 8: Drop rounded tablespoons of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Step 9: Bake for 10-12 minutes, or until the edges are set and the centers still look slightly soft and fudgy. Do not overbake!
Step 10: Remove from the oven and let the cookies cool on the baking sheets for 5-10 minutes before transferring them to a wire rack to cool completely. They will firm up as they cool.