Butter an 8-inch (20-cm) square baking dish and line with enough parchment paper that it comes up the sides. Butter the parchment paper, then place the baking dish on a heat-proof surface and set aside.

In a glass measuring jug, combine the heavy cream, butter, and salt, and microwave until the butter melts. (You can also do this in a small saucepan on the stove.) Set aside.

Place the sugar and water in a small, high-sided saucepan. Clip a candy thermometer to the pan and cook over medium heat without stirring.

Once the sugar has dissolved and the mixture is boiling, watch the temperature carefully (but do not stir the mixture).

After 5 to 10 minutes, the sugar will start to turn a deep amber. As soon as the temperature reaches 320°F (160°C), immediately turn off the heat and carefully pour the cream mixture into the sugar. (It will bubble up and steam.)

Turn the heat back on to low. Stir until the mixture is dissolved then stop stirring. Continue to cook for another 5-10 minutes until the mixture reaches 250°F (120°C), then immediately pour the hot caramel into the prepared baking dish. Let cool and set completely, at least 4 hours.

Once set, sprinkle with flaky salt, if using, and cut the caramel into 16 squares.

Individually wrap each caramel in a square of wax paper to keep them from sticking together and store in an airtight container at room temperature for 3 days.