Generously butter a cookie sheet.
Put butter, sugar, and water in a heavy pan or skillet over medium-high heat. Bring to a bubbling boil, stirring constantly with a wooden spoon, about 10 minutes. Remove spoon from pan, and cook to a very brittle stage (300 degrees to 310 degrees F on a candy thermometer). Or, make a cold water test: candy will separate into hard, brittle threads when dropped in cold water. Remove from heat and add nuts to mixture. Add vanilla and salt. Pour onto prepared cookie sheet and spread to 1/4-inch thickness. Cool slightly, top with chocolate chips or chocolate bars, and spread as it melts. Cool completely and break into pieces. Store in an airtight container.