Preheat the oven to 350 degrees and spray a 10-inch ovenproof skillet with nonstick spray.
Combine the melted butter, brown sugar, vanilla, and cocoa powder in a large bowl. Stir in the egg until combined. Add the flour, baking soda, and salt.
Transfer the batter to the prepared skillet and sprinkle with the chocolate chips. Bake until just set in the center and pulling away from the sides of the skillet, about 20 to 22 minutes. Let cool 10 minutes and loosen the edge. Turn out onto a wire rack to cool. Slice and serve.