1 1/2 cups well-stirred tahini, room temperature
1/2 tsp. kosher salt
1/2 tsp. vanilla extract
1 1/4 cups sugar

Line an 8x8" baking pan or a 9x4" loaf pan with parchment paper, leaving a 2" overhang on 2 sides.
Using a wooden spoon, mix tahini, salt, and vanilla in a medium heatproof bowl.
Pour 1/4 cup water into a small saucepan, then stir in sugar.
Attach a candy thermometer to side of pan.
Bring sugar to a boil over medium-high heat and cook, stirring occasionally to help sugar dissolve, until thermometer registers 250°F.
Immediately remove syrup from heat.
Gradually stream syrup into tahini mixture, mixing tahini constantly with wooden spoon.
Continue to stir just until halva comes together in a smooth mass and starts to pull away from sides of bowl (less than a minute).
Be careful not to overmix or halva will be crumbly.
Working quickly, scrape halva into prepared pan, pushing toward edges; smooth top.
Let cool to room temperature, then cover tightly with plastic and chill at least 2 hours.
Remove from pan using parchment overhang.
Slice as desired to serve.

For Pistachio-Rose Halva
Place a rack in middle of oven; preheat to 300°F.
Spread 1 cup unsalted shelled pistachios on a rimmed baking sheet.
Toast until fragrant and lightly golden, about 10 minutes.
Transfer nuts to a plate. Let cool, then coarsely chop.
Prepare halva, adding half of the chopped pistachios and 1/4 tsp. rosewater to the tahini, salt, and vanilla mixture.
Stir sugar syrup into tahini mixture and form halva as directed.
Sprinkle remaining nuts over, pressing gently to adhere.
Cover and chill as directed.
Cardamom-Chocolate Halva:
Prepare halva, adding 1/2 tsp. ground cardamom to the tahini, salt, and vanilla mixture.
Stir sugar syrup into tahini mixture, then quickly stir in 1/2 cup coarsely chopped dark chocolate just to combine (you want to see streaks of chocolate).
Form halva as directed. Sprinkle 1/4 cup coarsely chopped dark chocolate over, pressing gently to adhere.
Cover and chill as directed.
Lemon–Poppy Seed Halva:
Prepare halva, adding 1 Tbsp. poppy seeds to the tahini, salt, and vanilla mixture.
Add 1 Tbsp. finely grated lemon zest to water in saucepan before making the sugar syrup, then prepare syrup as directed.
Form halva as directed.
Sprinkle 1 Tbsp. poppy seeds over, pressing very gently just to adhere.
Cover and chill as directed.
Chocolate-Za’atar Halvah:
Prepare halva, adding 1/2 tsp. za’atar to the tahini, salt, and vanilla mixture.
Stir sugar syrup into tahini mixture as directed, then quickly stir in 1/2 cup coarsely chopped dark chocolate just to combine (you want to see streaks of chocolate).
Form halva as directed.
Sprinkle 1/4 cup coarsely chopped dark chocolate over, pressing gently to adhere.
Cover and chill as directed.