Line a 22 x 18cm tray with baking parchment. Put the butter and 150ml water in a pan over a low heat, and melt the butter (don't let the water boil). Tip in the sugar and dissolve, stirring often.
Bring the water to the boil, turn down to a simmer, then add the condensed milk. The mixture will be a light creamy colour.
Cook for 20-30 mins, stirring often, until the mixture is thick and a deep caramel colour.
To test, drop a little mixture onto a cold side plate, it should not run and should be thick and sticky after 30 seconds. Remove from the heat and beat for 10-15 mins until it has slightly thickened. Pour into the prepared tin and leave to cool for 30 mins before cutting into squares.