Preheat the oven to 350°F. Spray a 9- x 13-inch baking pan with baking spray, then line the pan with parchment paper leaving a 2-inch overhang (for ease in lifting out the brownies.
In a medium-sized microwave-safe bowl, heat the butter for 30 seconds on HIGH, stir, and then heat another 30 seconds until melted. You can also melt the butter in a saucepan set over low heat instead of using the microwave.
Add in the brown sugar and stir to combine, then return to the microwave for another 30 seconds for the brown sugar to completely dissolve. Don't worry if it separates, just stir it briefly to recombine. Alternatively, return the saucepan until it’s just hot, about (about 110°F to 120°F) but not bubbling.
Stir in the cocoa powder, baking powder, espresso powder (if using), and vanilla extract.
Add the eggs, stirring the batter until fully combined. Stir in the flour, then the chocolate, stirring until just combined. Transfer the batter to the prepared pan.
Bake the brownies for 30 to 35 minutes, rotating halfway through the baking time. They’re done when the edges appear firm and the center is set. A cake tester or toothpick stuck in the middle will come out with a few crumbs, but not raw batter.
Let the brownies cool in the pan set on a wire rack for at least an hour. Towards the end of the cooling time, you can put the pan in the refrigerator to help speed the process.
Cut into squares, serve, and enjoy!
Choose either regular unsweetened cocoa powder or double dark chocolate powder. The latter will give you a stronger chocolate flavor. Whichever you choose, just make sure it's Dutch process, not natural-style.Adding espresso powder to the brownie batter helps deepen the chocolate flavor. It’s optional, but highly recommended.You can use any combination of white chocolate, milk chocolate, semisweet chocolate, and dark chocolate chips or chunks in any proportion you prefer. Just keep the total to about two cups.This recipe can be halved and baked for 28 minutes in an 8- x 8-inch baking pan. It’ll make 16 squares.The brownies can be stored at room temperature for 4 to 5 days. The texture will be firmer on the second day.