Cook sugar and evaporated milk in a large saucepan over medium-high heat to a soft firm ball stage. (A drop of mixture dropped into a glass of cool tap water will hold the shape of a soft ball and have a firm but not hard texture.) Add butter and peanut butter and stir a little. Remove from heat and stir well. Whip with a large spoon then pour into a buttered 3x9 pan. Cut after somewhat cooled.