In 3-quart casserole or large glass mixing bowl, combine sugar and milk; blend well.
Microwave at high (10) 6 to 8 minutes until sugar is dissolved.
Stir every 2 minutes.
Add semisweet chocolate chips, milk chocolate chips, walnuts, butter and vanilla.
Stir until thoroughly blended.
Pour mixture into buttered 2-quart oblong glass dish.
Refrigerate until firm.