Heat the oven to 350°F (175°C). Line a 9 x 13-inch baking pan with parchment paper, leaving enough overhang on the long sides to lift out the cooled brownies.

Melt the butter in a saucepan over low heat or in a microwave-safe bowl. While the butter is still warm, whisk in the cocoa powder, sugar, salt, and baking powder until the mixture looks smooth and glossy.

Add the room-temperature eggs one at a time, whisking well after each addition. Stir in the vanilla extract until evenly blended.

Add the flour and fold just until no dry streaks remain. Avoid overmixing, since too much stirring can make the finished brownies less tender.

Fold in the semisweet chocolate chips or chopped chocolate. Add the chopped walnuts or pecans as well if you want a crisp, nutty contrast.

Spread the batter evenly in the prepared pan. Smooth the surface with a spatula so the brownies bake to a consistent thickness.

Bake for 30–35 minutes, until the edges are set and a toothpick inserted near the center comes out with moist crumbs rather than wet batter. Do not wait for the toothpick to come out completely clean.

Cool the brownies completely in the pan before cutting. For especially neat slices, refrigerate the cooled brownies for 30–60 minutes, then lift them out by the parchment overhang and cut into about 24 squares.