Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper.
Melt butter in a microwave-safe bowl. Stir in granulated sugar and brown sugar until smooth.
Whisk in eggs and yolk, one at a time. Stir in vanilla.
In a separate bowl, whisk flour, cocoa powder, salt, and baking powder.
Fold dry mix into wet ingredients until just combined. The batter will be thick.
Stir in chocolate chips, saving a few to sprinkle on top.
Spread batter evenly in the prepared pan.
Bake for 28–32 minutes, until the edges are set and a toothpick comes out with moist crumbs.
Cool completely in the pan, then lift out and slice into squares.