In a large bowl, mix sugar and cocoa well. In a saucepan, bring water to boil over high heat. Reduce heat to medium and add cocoa mixture. Cook to soft-ball stage of about 10 minutes. Reduce heat to low and add butter and vanilla. Stir until butter melts. Remove the pan from the heat and quickly add peanut butter. Stir just until blended. Pour immediately into a greased 9x9x2-inch baking pan. Score 1-inch squares while warm. Let cool about 20 minutes and cut when firm.