Combine butter, sugar, corn syrup and water in 2 quart microwave-proof bowl.  Micro-cook at HIGH 4 minutes; stir.  Micro-cook at HIGH 8 to 11 minutes or to 300° F. stirring every 3 to 4 minutes.  Stir in vanilla.  Pour into ungreased 13 x 9 x 2 inch baking pan.  Let stand to harden.  Break into chunks.  Melt chocolate in top of double boiler over (1 inch water) over low heat.  Chocolate temperature should be about 90° F. for dipping.  Coat chunks of toffee with melted chocolate and place on waxed paper; refrigerate to harden.  Re-dip candy and roll in pistachios.  Place on waxed paper; refrigerate until firm.