Line bottom and sides of 9-inch square pan with plastic wrap; set aside. Melt butter in medium-sized heavy saucepan over medium-high heat. Add agave, cream and brown sugar; mix well. Cook over medium-high heat until mixture comes to boil, stirring frequently. Reduce heat to medium and continue boiling, stirring frequently, until candy thermometer registers 250° to 255°F, about 45 minutes. Remove from heat and stir in vanilla; pour into prepared pan. Let cool completely in refrigerator before cutting into individual caramels with very sharp knife. Roll in chopped nuts or coconut, if desired, and wrap each individually in clear plastic wrap. Store, tightly wrapped in refrigerator up to 1 month. Caramels will be soft at room temperature and firm if kept chilled.