LINE 8- or 9-inch square pan with wax paper, extending paper over edges of pan.
MELT chocolate chips with sweetened condensed milk and vanilla in heavy saucepan over low heat, stirring frequently.
REMOVE from heat. Add peanut butter chips; stir just to distribute chips throughout mixture. Spread evenly into prepared pan.
CHILL 2 hours or until firm. Remove from pan by lifting edges of wax paper. Cut into squares.