Grease a 9x13 baking pan. You can also line it with parchment if you prefer for easy removal from the pan.
Combine the melted butter, brown sugar, and granulated sugar in a large mixing bowl. Whisk until the mixture is smooth and the sugar mostly dissolves into the butter.
Whisk in the eggs and vanilla.
Gradually sift in the flour, cocoa powder, baking soda, and salt - mixing in until just combined.
Fold in the chocolate chips.
Spread the batter into the prepared baking pan.
Bake for 25-30 minutes or until the top is set. (toothpick test should return mostly clean with a few moist crumbs)
Cool to room temperature before removing from the pan and serving.