• Line a loaf pan or other mold with plastic wrap leaving a generous overhang on each side. Set aside.
• Place the water, sugar, and salt in a medium sauce pan and bring to a boil, stirring occasionally to make sure the sugar is all dissolving. Once the mixture comes to a boil do not stir it again. Place a candy thermometer in the sugar mixture and cook to 248 Fahrenheit. While it is cooking use a pastry brush dipped in water to periodically brush down the sides of the pan to wash down any sugar crystals.
• While the sugar syrup is cooking place the tahini in a small sauce pan and heat to 120 Fahrenheit. (If you do not have a second thermometer just heat it until it is quite warm to the touch but not hot.) Stir frequently to prevent it from burning. Transfer the warm tahini to the bowl of a stand mixer fit with a mixing paddle. Add the vanilla extract.
• Once the sugar has come to temperature, immediately pour it into the mixing bowl. Turn it on the lowest setting and mix just until the mixture starts to look a bit grainy and loses some of its shine. Be careful not to overbeat the mixture or it will set in the bowl yielding a crumbly mess rather than a creamy finished candy. This should take no more than a minute. Immediately add the chocolate and mix just until the chocolate is swirled through. Working quickly, transfer the mixture to the prepared mold. Smooth down the top with a spatula and fold the plastic wrap over the top.
• Let cool at room temperature until set, approximately one hour. Store, well wrapped in plastic, in an airtight container in the refrigerator

Variations:

• Vanilla Halvah: simply omit the chocolate.
• Pistachio Halvah: omit the chocolate and add 1- 11/2 cups toasted unsalted pistachios to the mixing bowl with the tahini before adding the sugar syrup.
• Hazelnut: add 2 cups hazelnuts, toasted & skinned. You can buy roasted hazelnuts or spread raw hazelnuts in a single layer on a rimmed baking sheet and bake at 350ºF for 10 minutes. Transfer to a clean kitchen towel, folding the towel over the nuts and rubbing vigorously to remove the skin. Don't fuss if all the skins don't rub off - they never do! Finely chop into a breadcrumb size paste using a small food processor. combine hazelnuts, tahini, and vanilla. Stir until well mixed