Combine the milk, corn syrup, sugar and chocolate in a heavy 2-quart saucepan. Cook over medium heat, stirring constantly, until the sugar is dissolved. Continue cooking over medium heat until the candy reaches the soft ball stage (236 degrees F) on the candy thermometer. Stir occasionally to prevent sticking. Remove from the heat and add the butter. Cool to 110 degrees F without stirring, then add the vanilla. Beat until the fudge begins to lose its shine. Stir in the peanuts and quickly pour into a greased 8-inch square baking pan. Cut into squares when candy is completely cooled. Makes 1 1/2 pounds