Put the sugar, butter, water and corn syrup in a large, heavy-based pan. Stir over low heat until the sugar has dissolved.
Attach a sugar/candy making thermometer to the pan. Bring slowly to a boil and cook, for 10-15 minutes, without stirring, until the temperature reaches 250°F. If you do not have a sugar thermometer, test the toffee by dropping a small amount into cold water. It is ready when it reaches the soft crack stage (that's when it separates into threads that are hard and brittle).
Remove the pan from the heat and stir in the chocolate. Stir gently until melted, then pour immediately into the prepared pan.
When the toffee is starting to set, mark into squares with a sharp knife. Allow to set, then cut into squares when cold.