Line an 8- x 8-inch baking pan with parchment paper.
Combine sugar and vanilla with 1/2 cup water in a non-stick pot over medium heat, stirring to dissolve sugar. Allow mixture to simmer into a syrup, without stirring, until temperature registers 245 degrees on candy thermometer.
While syrup is cooking, place tahini and salt in bowl of a stand mixer fitted with paddle. Beat on medium speed and carefully stream syrup into tahini. Mix until syrup is incorporated and mixture begins to pull away from sides of bowl, about 30 seconds to 1 minute. Be careful not to overmix.
Transfer the mixture to the prepared pan and flatten it with a spatula; next, place another piece of parchment paper on top and smooth out the halvah with your hands.
Let he halva cool and come to room temperature, then cut into squares.
Store wrapped in plastc, inside a tin box, for a week or so.