Preheat the oven to 350°F (175°C) and grease a 9×13 inch baking dish.

Melt the butter in a saucepan over low heat; let cool slightly.

In a mixing bowl, combine melted butter, granulated sugar, and brown sugar until well mixed.

Beat in eggs one at a time, followed by vanilla extract.

In another bowl, whisk together cocoa powder, flour, and salt; fold this into the wet mixture until just combined.

Stir in chocolate chips if using and spread the batter evenly in the prepared dish.

Bake for 20-25 minutes until a toothpick inserted comes out with moist crumbs.