Adjust the oven rack to the lower third position, just below the center of the oven, and heat the oven to 350°F. Line an 8x8-inch pan with parchment paper or aluminum foil, leaving a 1-inch overhang on all sides. Spray lightly with nonstick spray and set aside.
In a medium heatproof bowl set over a pot of barely simmering water, melt the chocolate and butter, stirring constantly until the mixture is smooth. You can also microwave in 30-second intervals, stirring between each one.
Remove from heat and whisk in the cocoa powder. Set aside to cool slightly.
In a separate bowl, whisk the egg and sugar, vanilla, and salt together until well combined and the mixture is frothy.
Add the warm chocolate mixture and whisk until well combined.
Add the flour and use a silicone spatula to fold the batter together until no visible streaks of flour remain.
Spread the mixture evenly into the prepared pan and bake for 20-25 minutes or until a toothpick inserted into the center comes out with a few moist crumbs clinging to it.
Cool on a wire rack to room temperature, about 2 hours, then remove from the pan and cut into squares.
Notes
This recipe doubles easily. Bake in a 9x13-inch baking pan for about 35 to 40 minutes.
Chocolate: You may use any type of baking chocolate. I find semisweet chocolate pairs well with the unsweetened cocoa powder but you may wish to try bittersweet chocolate instead.
Cocoa powder: You may use regular cocoa or Dutch-processed.
Flour: Make sure to measure the flour correctly! Accidentally using too much flour will cause the brownies to be dense and hard.
Make ahead tip
Brownies can be stored at room temperature in an airtight container for up to 1 week.
Brownies can be frozen for up to 3 months. Thaw in the refrigerator overnight and bring to room temperature before serving.