Preheat oven to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper.
Melt butter, whisk with sugar and brown sugar until smooth.
Add eggs and vanilla, whisk until combined.
Sift in flour, cocoa, baking powder, and salt. Fold gently.
Stir in chocolate chips if using.
Pour batter into pan, spread evenly, and bake for 25–30 minutes.
Cool completely before slicing into squares.