In a heavy bottomed skillet (NOT a non-stick skillet), over medium heat, combine the butter, sugar, water and salt. As the butter melts, stir with a wooden spoon or spatula to combine.
Set a brown paper bag near the stove and let the mixture boil until it matches the bag in color. This takes between 18-20 minutes and I ALWAYS set a timer. Just in case I find myself distracted, the timer prevents me from letting the candy burn.
While the toffee is boiling, prepare two large baking sheets by lining them with parchment paper. (Do not use waxed paper, it will melt.) The pans will be extremely hot when you pour the toffee onto them, so set them on hot pads or a folded towel on top of the counter.
The moment the toffee is caramel colored, remove from the heat and add the vanilla. Stir to combine. Quickly divide the toffee between the two baking sheets. Using a spatula or an offset knife, spread the toffee as thinly as possible across the baking sheets. Let it cool completely.
You can set the pans in the refrigerator to speed up the process, however, room temperature will work fine too. When the toffee is cool, blot with a paper towel to remove excess oils from the surface.
Place the chocolate in a glass bowl and melt in the microwave at 50% power for 60-120 seconds, stirring every 30 seconds or so, until smooth.
Spread the warm chocolate across the toffee and then place in the refrigerator to cool completely. Sprinkle with Maldon salt, if desired. Once cooled, break into pieces. Store in an airtight container in the refrigerator.