Wash and dry apples. Remove stems. Insert one wooden stick into the stem end of each apple. Set apples aside.
Melt butter and unsweetened chocolate in a heavy 3-quart saucepan over low heat. Add brown sugar, corn syrup, and sweetened condensed milk; mix well.
Clip a candy thermometer onto side of pan. Cook over medium heat, stirring frequently, until thermometer registers 245 degrees (firm-ball stage). Remove saucepan from heat. Stir in vanilla.
Dip each apple into the hot caramel mixture, and use 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 parchment or waxed paper and let stand about 25 minutes or until firm. Makes 18.