Preheat oven to 350°F (175°C). Grease or line an 8×8-inch baking pan with parchment paper.

In a medium bowl, mix melted butter, sugar, eggs, and vanilla until well combined.

Sift in cocoa powder, flour, baking powder, and salt. Stir gently until just combined—don’t overmix.

Fold in chocolate chips or nuts if using.

Pour batter into the prepared pan and spread evenly.

Bake for 20–25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).

Let cool completely before cutting into squares.