Mix sugar and milk together in a sauce pan. Put on burner on high heat. Without stirring, bring to a full rolling boil. Immediately reduce heat to “low”. Cook exactly 5 minutes. Remove from heat. Add peanut butter and vanilla. Beat quickly until thick and immediately pour into a well buttered or well sprayed pan. Use an 11 ¼ X 7 ¼ pan (cookie sheet type pan) for thinner squares, or an 8 X 8 pan for thicker fudge.