Combine the sugar, milk and corn syrup in a heavy 3-quart saucepan and bring to a boil, stirring constantly, until the mixture boils. Continue boiling without stirring to the soft-ball stage (243 degrees F on the candy thermometer). Remove from the heat. Add the butter without stirring and cool to lukewarm, 110 degrees F. Add the vanilla and peanut butter then beat until the mixture begins to thicken and lose its gloss. (Watch carefully as this fudge has a short beating time.) Quickly add the chocolate pieces and turn into a 8 X 8 X 2-inch pan. While warm, mark into squares. Cool until firm then cut as marked. Makes about 3 dozen squares.
