Combine sugar, milk and corn syrup in a heavy saucepan (3 quart); bring to a boil, stirring constantly, until mixture boils. Continue boiling without stirring to the soft-ball stage (234 F). Remove from heat. Add butter, without stirring, and cool to lukewarm (110 F).
Add vanilla and peanut butter; beat until mixture begins to thicken and lose its gloss. (Watch carefully, this fudge has a short beating time.) QUICKLY add chocolate pieces, and turn into buttered 8x8x2 inch pan. While warm, mark into squares. Cool until firm, then cut as marked.