Combine all ingredients in a medium saucepan, and stir over low heat until sugar is dissolved.
Increase heat and bring to a boil.
Put candy thermometer in place and continue cooking without stirring. During cooking wash, sugar crystals from sides of pan with a pastry brush dipped in water. Cook until mixture reaches 238 degrees Fahrenheit.
Remove pan to cooling rack and cool to lukewarm (about 110 degrees Fahrenheit), without stirring or moving the pan.
Beat vigorously with a wooden spoon or electric mixer until mixture begins to lose its gloss and is of spreading consistency. Ice cupcakes. If frosting becomes too thick, beat in a few drops of cream or milk.