Adjust an oven rack to the middle position and heat the oven to 350 Degrees.

Line an 8-inch square pan with foil and coat lightly with vegetable oil spray.

Melt the butter and cocoa powder together in the microwave, stirring often, for 1 to 3 minutes.

Let the melted chocolate mixture cool slightly.

Whisk the sugar, eggs, vanilla extract, baking powder, and salt together in a large bowl until combined.

Whisk in the melted chocolate mixture until smooth.

Stir in the all-purpose flour until no streaks remain.

Scrape the batter into the prepared pan and smooth the top.

Bake until a toothpick inserted into the center comes out with just a few crumbs attached, for 22 to 27 minutes.

Let cool completely on a wire rack to room temperature, about 2 hours.

Remove the brownies from the pan using the foil.

Cut into squares.