Blend cake mix, eggs and melted butter with electric mixer on low speed until the ingredients are moistened (about 1 minute). Scrape down sides and stir in chocolate chips, stirring until well incorporated. Mixture will be very thick.
Scrape mixture into 13x9 pan. Using fork or fingertips, press batter out evenly so that it reaches all the sides.
Bake until golden brown and a toothpick inserted in the center comes out with a few crumbs attached. Bake 20-25 minutes. Cool on wire rack for 30 minutes before cutting into squares.