In large saucepan, melt butter, brown sugar, cocoa powder and salt over medium heat, stirring often until completely liquid and very warm to the touch. Remove saucepan from heat. Quickly add eggs and vanilla and stir until combined. Add flour and baking powder; stir until batter is very shiny and combined. Mixture will still be very warm. Add chocolate chips, stirring as the chips melt. Pour into prepared pan.