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Preheat the oven to 350°F with a rack in the center. Lightly grease an 8″ square loaf pan. Line the pan with a parchment sling and lightly grease the parchment.
In a large saucepan or microwave-safe bowl, heat the butter over medium-low heat or microwave for 45 to 60 seconds, covered, on 50% power, until just melted.
Whisk the sugar into the butter. Add the egg and vanilla and whisk until the mixture is shiny and emulsified.
Whisk in the cocoa and espresso powder until well combined.
Add the flour, salt, and baking powder and whisk until just combined.
Using a flexible spatula, stir in the chocolate chips until incorporated.
Transfer the batter to the prepared pan and spread it evenly to the edges.
Bake the brownies for 30 to 35 minutes, until the edges feel set and a toothpick or a sharp knife inserted into the center comes out with moist crumbs clinging to it but no raw batter. A digital thermometer inserted in the center should register 205°F when the brownies are done.
Remove the brownies from the oven and let cool completely in the pan on a rack. Use the parchment to lift the brownies out of the pan and transfer them to a cutting board before slicing.
Store leftover brownies in an airtight container at room temperature for several days; freeze for longer storage.