In a medium bowl, combine half of the butter with the cake mix and egg. Work the mixture with your hands until everything is well blended. The mixture will be crumbly. Press the mixture evenly into a 25x38cm swiss roll tin.
Bake for 10 minutes in the preheated oven. Remove and cool for 10 minutes.
In a saucepan, combine the remaining butter with the evaporated milk and toffees, over medium heat. Stir occasionally until smooth. Remove from heat and pour the mixture over the baked base, turning the tin from side to side so that it is evenly coated.
Return to the oven and bake for 15 to 20 minutes, until it is bubbly all over. Let cool.
Meanwhile, make the icing, stir together the icing sugar with the water and vanilla until smooth. If the icing is too thick, add more water. If it is too thin, add more icing sugar. Drizzle over bars. Let set before cutting into squares.