Melt 1 cup chips & butter in large, heavy pan over low heat; stir until smooth. Remove from heat. Add eggs; stir well. Add flour, sugar, soda & vanilla; stir well. Stir in remaining 1 cup chips & nuts. Spread into 13X9" baking pan. Bake in preheated 350 oven for 18-22 minutes or until wooden toothpick inserted in center comes out still slightly sticky. Cool completely. Cut into 2" squares.