In a medium-sized bowl or standing mixer, combine butter and sugar. Stir in vanilla and eggs until fully incorporated. Mix in cocoa powder, flour and salt until smooth. Gently fold in chocolate chips. Spread evenly into prepared pan, batter will be thick. Bake for 20 to 25 minutes, until toothpick comes out with only a few moist crumbs. Cool in pan for a few minutes, then transfer to wire rack to cool completely.