In a 3 quart saucepan, combine white sugar, evaporated milk, and cocoa. Bring to a hard boil, and then reduce heat to medium. Continue cooking until it reaches the soft ball stage, 234 degrees F (112 degrees C).
Stir in peanut butter until well blended. Pour mixture into a buttered 8x8 inch baking dish. Cool, and cut into pieces.
