Ingredients:

  Amount  Measure       Ingredient -- Preparation Method
--------  ------------  --------------------------------
   1      cup           balsamic vinegar
   4      centiliters   garlic -- sliced
   4      pinches       -- (2-inch) fresh
                        -- rosemary
   2      tablespoons   butter -- optional

Preparation:

Put all the ingredients except the butter into a saucepan and bring to a boil. Reduce the heat and simmer until it is reduced by half, about 10 minutes. The sauce is ready when it coats the back of a spoon. Add the optional butter and stir well.