Wash and dry apples. Remove stems. Insert one wooden stick into the stem end of each apple. Set apples aside.
In a heavy 3-quart saucepan melt the butter or margarine and unsweetened chocolate over low heat. Add brown sugar, corn syrup, and sweetened condensed milk, then mix well. Clip a candy thermometer onto side of pan. Cook over medium heat, stirring frequently, till thermometer registers 245° (firm-ball stage). Remove saucepan from heat. Stir in vanilla.
Dip each apple into the hot caramel mixture, using a spoon to spread the mixture evenly over apple. Allow excess caramel mixture to drip off. Immediately dip bottoms of apples into peanuts. Place apples, peanut side down, on waxed paper and let stand about 25 minutes or till firm.