Preheat oven to 350°F. Spray an 8-inch square baking pan with cooking spray and line it with parchment paper long enough to allow the ends to extend over the pan.
In a large mixing bowl, add hot water and espresso powder. Whisk until the powder is dissolved.
Add melted butter, eggs, granulated sugar, and vanilla. Stir until the ingredients are incorporated.
In a separate bowl, whisk together the flour, confectioners' sugar, cocoa powder, and salt.
Add the flour mixture to the butter mixture, mixing until just combined.
Fold in ¾ cup of the chocolate chunks, reserving the rest for the topping.
Spread the batter into the prepared pan. Top with the remaining chocolate chunks.
Bake the brownies for 20-23 minutes. Brownies are done when an inserted toothpick is removed with crumbs but not wet batter.
Let the brownies cool in the pan before transferring them to a wire rack to cool completely.