LINE 8- or 9-inch square pan with wax paper, extending paper over edges of pan.
MELT peanut butter chips with 1 cup sweetened condensed milk, butter and salt in heavy saucepan over low heat.
MELT chocolate chips with remaining sweetened condensed milk in small saucepan. Remove from heat; add vanilla. Stir until smooth.
SPREAD peanut butter mixture evenly into prepared pan. Spoon chocolate mixture over peanut butter mixture. With table knife or metal spatula, swirl through top of fudge.
CHILL 3 hours or until firm. Remove from pan by lifting edges of wax paper. Cut into squares.