In a heavy 3-qt. saucepan, combine the butter, brown sugar, milk, corn syrup and water; bring to a boil over medium heat. Cook and stir until a candy thermometer reads 230° (thread stage), 8-10 minutes. Remove from the heat; stir in vanilla.
Transfer to a small fondue pot or 1-1/2-qt. slow cooker; keep warm. Serve with apple slices.
We recommend that you test your candy thermometer before each use by bringing water to a boil; the thermometer should read 212°. Adjust your recipe temperature up or down based on your test.