Step 1: Preheat your oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper, leaving a slight overhang on the sides for easy removal.
Step 2: In a large mixing bowl, combine the melted butter, granulated sugar, and brown sugar. Whisk vigorously for about 1 minute until well combined.
Step 3: Add the eggs and vanilla extract. Whisk for another 2 minutes until the mixture is light in color and slightly fluffy; this creates the crackly top.
Step 4: Sift in the cocoa powder, flour, salt, and espresso powder. Use a rubber spatula to gently fold the dry ingredients into the wet until just barely combined. Do not overmix.
Step 5: Fold in the chocolate chips if using, then pour the batter into the prepared baking pan and smooth the top with your spatula.
Step 6: Bake for 22-25 minutes. A toothpick inserted into the center should come out with a few moist crumbs attached (not liquid batter).
Step 7: Remove from the oven and allow to cool completely in the pan for at least 30 minutes before lifting the parchment to slice.