Preheat your oven to 350°F (175°C). Lightly grease a 10x15-inch jelly roll pan or quarter sheet pan, and line it with parchment paper if you’d like easy cleanup and smooth edges.
In a medium saucepan, melt 1½ cups of unsalted butter over low heat. Once melted, remove from heat and stir in 2 cups of semi-sweet chocolate chips until smooth and glossy. Set it aside to cool slightly.
In a large mixing bowl, whisk together ½ cup granulated sugar, 1 cup brown sugar, and 5 large eggs until well combined and slightly fluffy, abiove 1 to 2 minutes. This helps give the brownies their classic texture.
Whisk in 2 tablespoons of vanilla extract, then slowly pour in the warm (but not hot) chocolate-butter mixture. Stir until fully combined and smooth.
In a separate bowl, mix together 1½ cups all-purpose flour, 2 teaspoons baking powder, and ½ teaspoon salt. Gently fold the dry ingredients into the wet mixture using a spatula. Be careful not to overmix—you want everything just combined.
If you're using them, fold in an extra 1 cup of chocolate chips for melty pockets of chocolate in every bite.
Pour the batter into your prepared pan, spreading it out evenly. Bake for 25 to 30 minutes, or until the top is shiny and set, and a toothpick inserted near the center comes out with just a few moist crumbs (not wet batter!).
Let the brownies cool in the pan on a cooling rack for at least 20 minutes. Once they’ve set, slice into 18 to 24 squares, depending on your preferred size.