Mix the cake mix, margarine and egg together and put 2/3 of the mixture into a greased 9 x 13" pan. Pat down to cover bottom of pan.
Mix together: chocolate chips, sweetened condensed milk, margarine, and pinch of salt. Pour over the dough. Crumble the remaining 1/3 dough mixture on top of the chocolate. It should look marbled. Bake at 350 degrees for 25 minutes. YUM!