Preheat oven to 350 degrees Fahrenheit. Lightly grease a 9x13-inch baking dish and set aside.
In a mixing bowl with a hand mixer or bowl of stand mixer with the paddle attachment, combine melted butter, brown sugar and white sugar.
Add eggs and vanilla and mix for another 2 minutes.
Add salt and baking powder and mix until combined.
Add cocoa powder and flour and mix until just combined.
Fold in the chocolate chips.
Pour the batter into the prepared baking dish and smooth out until even.
Bake for 25–30 minutes, or until the middle no longer jiggles.
Let sit in the pan for 10–15 minutes and let cool completely before cutting.