Preheat oven to 350°F. Grease or line a 9x13-inch baking pan with parchment paper.
Melt chocolate and butter together in a microwave-safe bowl in 30-second intervals, stirring until smooth.
Add sugars to the melted mixture and stir until combined.
Mix in eggs and vanilla, stirring until smooth and glossy.
Add flour, baking powder and salt. Stir just until combined. Do not overmix.
Pour batter into prepared pan and spread evenly.
Bake for 30–35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Cool completely before cutting for clean slic