Melt chocolate and butter in large heavy saucepan over very low heat. Add sweetened condensed milk; stir to blend. Remove from heat.
Combine granulated sugar, water and corn syrup in small saucepan. Cook over medium heat, stirring constantly, until sugar is dissolved. Cook, without stirring, until mixture reaches 250° F on candy thermometer or until a small amount of syrup, when dropped into very cold water, forms a firm ball which does not flatten when removed from water. (Bulb of candy thermometer should not rest on bottom of saucepan.) Remove from heat; stir into chocolate mixture. Add vanilla, coconut and nuts; stir until well blended.
Refrigerate about 30 minutes or until firm enough to handle. Shape into 1-inch balls; roll in powdered sugar. Store tightly covered in cool, dry place.Makes about 4 dozen candies