Add the sugar, butter, and water to a medium pot.
Heat over medium heat until the sugar has dissolved and butter has melted.
Add a candy thermometer to the pot (make sure the thermometer part doesn't touch the bottom).
Maintain a medium to medium-high heat, I have mine at almost 6 out of ten. Don't stir, just wait through all of the stages. Mine hit soft crack stage at 14 minutes,
Once it's ready, remove it from the heat immediately, stir in vanilla if using and pour onto the pan. Do this quickly but carefully as the sugar will continue to cook.
Tilt the pan or use a small offset spatula to even the toffee out.
Let cool for 5 minutes or so. It'll start hardening on the outsides, this is fine, it's still hot enough to melt chocolate.
Sprinkle chocolate chips on. Let sit 5-10 minutes, or until melted.
Spread chocolate out with a small offset spatula. Sprinkle flaky sea salt if you like.
Chill to set chocolate and then break into pieces.