Mix sugar, half cup water, and optional cocoa in a saucepan, and boil rapidly for exactly one minute. Pour over peanut butter and vanilla in a bowl, and beat until thick. Pour into buttered pan and allow to cool. Cut into squares. This recipe starts out as peanut-butter fudge, but with the addition of powdered cocoa, can apparently been engineered to be chocolate. I've not tried the cholocate variation, but the basic pb is so good and so easy that its certainly worth the try...