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.
While the chocolate mixture is still warm, add the granulated sugar and brown sugar. Stir well to help the sugar begin to dissolve.
Add the 3 eggs, egg yolk, and vanilla. Whisk until the batter is smooth, thick, and glossy. This helps create that shiny brownie top.
Fold in chocolate chips, if using.
Spread the batter evenly in the prepared 9 x 13-inch pan.
Bake for about 25–30 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs rather than wet batter.
Cool completely before cutting for clean slic