Step 1: Preheat your oven to 350°F (175°C).
Step 2: Line your 9x13 inch baking pan with parchment paper, leaving some overhang on the sides.
Step 3: In a double boiler or microwave-safe bowl, combine the chopped semi-sweet chocolate and the cubes of unsalted butter. Melt together until smooth, stirring frequently. Remove from heat and let it cool slightly.
Step 4: In a separate mixing bowl, whisk together the all-purpose flour, salt, cocoa powder, and instant espresso powder until well combined. Set aside.
Step 5: In a large mixing bowl, combine the melted chocolate and butter mixture with the granulated sugar. Whisk until fully incorporated, then add in the lightly beaten eggs and vanilla extract. Mix until smooth.
Step 6: Gradually add the dry ingredients to the wet mixture, folding gently with a rubber spatula until just combined.
Step 7: Fold in the chocolate chunks, ensuring they are evenly distributed throughout the brownie batter.
Step 8: Pour the brownie batter into your prepared baking pan and spread it out evenly. Bake for 25-30 minutes, or until a toothpick inserted comes out with a few moist crumbs.
Step 9: Allow the brownies to cool in the pan on a wire rack for at least 20 minutes. Once cooled, lift them out and cut into squares.