Step 1: Preheat your oven to 350°F (175°C).
Step 2: Grease your baking pan with butter or line it with parchment paper.
Step 3: In a medium mixing bowl, whisk together the all-purpose flour, salt, and cocoa powder until well combined. Set aside.
Step 4: In a heatproof bowl, melt the chopped semi-sweet chocolate and cubed butter together over a double boiler, stirring occasionally until smooth.
Step 5: Stir in the instant espresso powder to the melted chocolate mixture until fully dissolved.
Step 6: In a large mixing bowl, whisk together the granulated sugar and lightly beaten eggs until pale and slightly thickened.
Step 7: Pour the melted chocolate mixture into the egg and sugar mixture, adding vanilla extract and mixing until well incorporated.
Step 8: Gently fold the flour mixture into the chocolate batter until just combined.
Step 9: Stir in the chocolate chunks until evenly distributed.
Step 10: Pour the batter into the prepared baking pan and smooth the top. Bake for 25-30 minutes.
Step 11: Allow to cool in the pan on a wire rack for at least 30 minutes before cutting into squares.
